You build a pan or reduction sauce like you do a house. In layers. You first set a foundation. For pan sauce, that foundation is made of some type of oil and if you are using bacon, lardons or pancetta, that means you would first saute off your seasoning meat and remove the seasoning meat from the pan but reserve the fat produced from cooking it.
Then you would use the fat to cook whatever portioned meat you are cooking: Pork chops, pork tenderloins, chicken breasts, chicken parts, flattened chicken or flattened game hen, beef, veal, etc ad nauseum. Then you would usually finish off the meat in the oven because it is an "all over heat" and you want that to cook the meat gently to retain the maximum amount of moisture. The reason you sear the meat in the fat first is three fold: 1. It adds pretty caramellized color. 2. It locks in the liquid juices inside the meat. 3. It adds the second layer or "frame" of seasoning to the pan that will eventually build the sauce.
The walls or structure of the sauce occurs after the portion meat is done and removed from the pan. That is when you use the fat and juices in the pan to cook off aromatics that will add body to the sauce. The aromatics can be anything from a mirapoix - French trinity (onion, celery, carrot), to Cajun trinity (onion, celery, bell pepper), to sofrito - Mexican/Spanish/Italian (onion, bell pepper, peppers). It can also be as simple as a clove or two of garlic or a diced shallot or a spoon of chopped onion or green onion. Whatever veggie aromatic you want to do. You cook these till tender and usually translucent but sometimes I want a deep pan sauce and I will caramellize them which means I will cook them longer.
Now it's time for the roof. You keep all the veggies in the pan and usually there won't be alot of extra fat but if there is, pour off the extra fat. You don't have to be exact about this. It's ok to leave a little bit. It will all bind in the end. But the roof is the liquid that will actually make up the sauce. You will deglaze the pan, so you want the pan to be piping hot. Again, all the elements are still in the pan. You are simply adding the next layer of flavor. I usually start by deglazing with a wine, or spirit. It adds unbelievable flavor and makes it complex. You cook it to reduce and concentrate the flavor but to also burn off a majority of the alcohol in it. The rule of thumb is to reduce by half. I rarely measure this amount. I do it till it looks right. But for beginners that's anywhere from 1/4 cup to 1/2 cup. I usually let it go a little farther than 1/2 way cuz I want a syruppy sauce at this point. It will only add more flavor. You can also deglaze using vinegars, stock, broth, cream, or water. If you are only using on liquid, then only reduce it to 1/2 or a bit less. If you're only using one layer, you will want this a little thinner and sauce-like rather than syrup-like.
NOTE HERE: If you are using a demi-glace or making a glace de viand, this is where you add the tablespoon or two of congealed concentrated stock. Let it dissolve and again, reduce the sauce.
The next part of the sauce is the drywall and fixtures. Add the next layer of liquid. For me the next layer is the layer of liquid like stock or broth. I usually make more stock based pan sauces than I do cream based...just out of preference. I will also add any seasoning herbs at this time like thyme, rosemary, oregano, parsley, peppers, etc. I let this layer cook down by 1/2 until it's the finished consistency I'm looking for in the sauce.
The next layer is the paint of the house. I add salt, any fine herb seasoning notes that I don't want to mute in the sauce. Any grated cheeses like gruyere or parmesan or anything like that. I do this part off the heat but still in the pan.
The last layer is the touch up work. Cuz the house is built. But before I can finish off the house, I have a decision to make. Am I making a rustic pan sauce where I will serve it "chunky"? Or am I wanting a professional, silky finish. If I'm making a silky sauce, what I'm doing (mainly for company), I will strain the sauce through a fine mesh strainer prior to adding the butter to finish it. If I am doing a non-cream based sauce, I will finish my pan sauce with a tablespoon or two of cold unsalted butter. I will first remove and stalks of herbs like thyme or rosemary and then I will swirl the butter into the sauce to incorporate it by shaking and swirling the pan. It must be done off the heat. I do this because it makes the texture silky.
Then the sauce is finished and I serve it immediately. From start to finish a pan sauce should take no more than the 5 minutes of resting that you give your meat. Most of mine are made in about 3-4 minutes. It's very quick!
This is the way I would do this recipe:
First I would saute the pancetta for the sauce in your saute pan that will be used for the chicken. Don't do it in a non-stick pan.
Then, remove the pancetta and leave any fat that rendered out. It probably won't be that much, so I would either supplement with extra bacon grease or extra pancetta grease.
Then use that fat to "sear" the stuffed breast. Sear it on all sides. I would probably season it on the inside of the meat. Then I would finish it in an oven at about 350 or 400 until cooked through. Check whatever the recommended internal temp is for chicken and sausages. I would guess it would be about 8-12 minutes in that range.
Then I would remove the breast from the pan to let it rest for 5 minutes prior to cutting and finish your sauce:
Add a half of a shallot and a clove of garlic, chopped very fine and saute quickly till tender (about 2-3 minutes).
Pour off any extra grease.
Deglaze pan with 1/3 cup of white wine or vermouth
Reduce wine and aromatics by 1/2
Add cream to pan
Reduce sauce by 1/2 or more.
The sauce will thicken as it reduces. Maybe add a chiffonade of basil at the very end. Just one or two leaves, rolled and sliced across the grain very thinly.
Adjust salt and pepper levels of sauce.
Slice breast into 3 or 4 1" slices on a diagonal and either place on a sauce or gently nappe the sauce over the meat.
Top with crumbled pancetta.
Eat and enjoy.