Steak is a passion of mine. I've been on a lifelong quest to perfect my steak cooking ability myself, and have ended up discovering that what matters the most is the cooking technique (creating a beautiful crust, reaching the right internal temp etc...) and the absolute perfect amount of seasoning.
1) That's a tough one. Many will say that a good steak only needs salt and pepper, and I often agree. I know some die-hard steak fans who'll tell you a great steak doesn't even need any salt or any pepper. I personally love salt and pepper on my steak, and use some just minutes before cooking the steak, and a bit more after the steak has been rested, sliced (if sliced), and plated. I don't like Kosher salt, and I prefer using fine sea salt (I like La Baleine) before cooking, and fleur de sel when plating. Freshly ground black pepper before and after cooking.
I love rosemary and have big rosemary bush in my backyard, but I would never use it on steak. If you're going to flavor a steak beyond S&P, I recommend an excellent quality butter, some fresh thyme and some garlic. If you pan fry or pan roast the steak then you can speed baste it with butter that infuses with the thyme and smashed garlic cloves. You can also use the thyme to brush the butter on the steak while it cooks. If you grill you can brush the steak with the thyme. Another thing you can do is add a pat of butter on top of the steak just before serving.
If you insist on rosemary then you could try just placing it on top of the steak while it's grilling, or placing it next to the steak on top of a small piece of aluminum foil. But... yeah I wouldn't go for rosemary personally.
2) I wouldn't use herb in a reduction for steak. Sometimes I make rather-advanced demi-glace-based pan sauce with cracked green pepper... but often my favorite is the simplest jus: after you remove the steak from the pan, and while it rests, return the pan to the fire, "pince" the "sucs", which in French means cook the fond, the caramelized bits at the bottom of the pan, then deglaze with water. Yes you can use wine, or white wine, or vermouth, or nearly any liquid that may strike your fancy, but trust me, water works great for this, and will leave the flavor of your steak intact, even in the reduction. Now a jus like that won't have body, it will be very thin, but if you do it right it will be very very dark brown, a bit salty and PACKED with concentrated steak flavor. There may not be much of it, but you don't need much of it, about a teaspoon or two per steak. Liquid gold.
3) Shallots are absolutely wonderful, much fancier than onions, and I would definitely use them if I made the demi-glace pan reduction sauce. I wouldn't caramelize them though, I don't need more sweetness on my steak, and I've got enough maillard reaction going on the steak's crust. I would just barely sweat them a bit and infuse them in whatever liquid I used to deglaze the pan for that sauce, which could be white wine, red wine, or even cognac.
4) Nothing better than water IMO. Less is more.
5) Huh... here you lost me. I suppose we're no longer talking about steak.
6) If you deglaze with cognac you can flambé it: let it heat in the pan a bit and tilt the pan so its vapors catch on fire from the flame of the stove. There are two goals: 1) to accelerate the burning off of the alcohol (it's often considered undesirable to have actual alcohol content in a finished sauce, although of course even that's debatable) and 2) to show off your mad skillz in front of your friends or in-laws or whoever you're showing off (huh... cooking) for.
One thing you didn't mention and that is paramount is to rest the steak after cooking but before slicing and plating. The larger the piece the longer it needs to rest. That really makes a huge difference in the end result.
Now look at what you've done: I have to go buy some bone-in rib-eyes!!!
Edited by French Fries - 11/1/14 at 10:13pm