Calf or veal liver are preferable. Beef liver is comparatively tough and bitter. Unless you are very good with a knife, it's a good idea to have the butcher slice it about 3/8" thick. This might mean going to an actual meat market -- depending on the level of service you get from your super.
Based on your short description you can't expect a duplication of the dish you had at Cap's. There's just not enough specificity. For instance, I don't know if bacon was served with the liver, or if there was something special about the bacon itself. Nevertheless, you did provide enough for me to have a pretty good idea of techniques and most of the ingredients required. Fortunately, there aren't a lot of right ways to do this.
It's important to use very fresh liver, and, as I said have it sliced fairly thin. The techniques involved are soaking the liver in milk to remove the bitterness; using medium sliced liver to maximize flavor and surface area; cooking only briefly so the veal is medium-rare to medium; making a simple, butter-finished pan reduction. Also, described is one of the better ways to cook bacon.
CALF'S LIVER IN A BALSAMIC PAN REDUCTION
2 cups milk
1 - 1-1/2 pounds fresh calf's or veal liver, sliced 3/8" - 1/2" thick
1/2 - 1 tsp vegetable oil
4 slices, thick sliced, American style, best quality bacon
2 cups onions, thin (or lyonnaise) sliced
2 tbs kosher salt
2 tsp freshly ground black or white pepper
2 tsp paprika
Flour, for dredging (about 3/4 cup)
2 tbs "everyday" balsamic vinegar
3/4 cup veal stock, or low sodium beef and chicken stock mixed 50/50.
2 tbs butter, divided
Rinse the liver in the sink, shake it dry and place it in a baking pan. Pour enough milk into the pan to completely cover the liver, and turn the liver in the milk. Cover the pan with cling wrap and reserve in the refrigerator for at least 45 minutes, and up to 24 hours.
About 45 minutes before you plan to eat, remove the liver slices from the milk, and dry them with paper towels. Set the liver on a plate. Mix the salt, pepper and paprika together in a small bowl, and use the rub to season the liver. Let the salt be your guide as to when to stop. Reserve the remaining seasoning.
Put the flour on a plate, and dredge the liver slices in it until well covered. Set the floured slices on a rack for the flour to set during the bacon and onion cooking processes.
Cut the bacon slices in half, crosswise so that four long slices become eight short ones. Then fry them as follows: Take the pan you're going to use to fry the veal, and while it's still cold add a little oil, about 1/2 tsp to 1 tsp depending on the size of the pan. Turn the heat under the pan to medium and add the bacon slices. As soon as the slices start to go limp, turn them; and when the pan starts to sizzle, turn them again. The oil and frequent turning will maximize the amount of fat rendered by the bacon. The frequent turning also helps keep the bacon rashers flat. When the bacon rashers are cooked to your preferred degree of crispness, remove them from the pan and set them on a paper towel to drain.
Drain the pan of all but 2 tbs of fat, and reserve the rest. Return the pan to the medium flame and add the sliced onions to the pan. Allow the onions several minutes to begin browning before tossing or stirring with a spoon. Continue to cook several minutes more, until the onions are browned all over. Remove them from the pan, and reserve.
Add an additional 2 tbs of bacon fat to the pan, and raise the temperature to medium high. When the fat is hot, add as much of the liver as the pan will hold without crowding. Allow the liver to brown before turning or disturbing in any way. About 2 to 3 minutes. Turn the liver, and brown the other side, about 2 minutes. Remove the liver, set it on a towel to drain, and reserve. If you have any remaining uncooked liver, cook it as you did the first batch. You may have to add additional bacon fat. When it is browned, remove, drain and reserve.
Drain the pan of most of the bacon fat, by pouring it out. Don't wipe out the pan. Set the pan back on the heat, add the balsamic vinegar, and deglaze the pan by scraping up all the cooked flour, bacon, onion and liver bits that stuck to the bottom during the cooking process. When the pan is deglazed, add the stock and reduce by about one third to nappe
consistency. Reduce the heat to low. Return the liver to the pan, just to warm it through.
Divide the onions between the plates, in small heaps. Lean one slice of liver against the mound and arrange the other slices as neatly as possible.
Add 1 tbs of butter to the pan, turn off the heat, and swirl the pan until the butter is incorporated into the reduction. As soon as it is, do the same with the remaining butter. Pour the sauce through a fine sieve, to give it shine and texture. Sauce the liver.
Serve with mashed potatoes if you must.
Hope this helps,