I have made many different variations of this dish, and when I did a quick Google search to see what the classic version was, it seems that all you really need for a dish to be called saltimbocca is chicken (or veal), sage, and prosciutto. You can make this as simple or as complicated as you'd like. Here's what I did earlier this week:
3 chicken breasts, cut into thirds, pounded to about 1/4 inch thick
1 package of proscuttio (8 slices?), cut in half
1 large shallot, sliced
2 cloves of garlic
~1 tsp dried sage, or 1 fresh leaf for each piece of chicken
8oz mushrooms (optional), sliced
2 T capers (optional)
1/3 c chicken stock
juice of 1 lemon
salt and pepper
Slice the chicken breasts in thirds on a diagonal, basically making the pieces as evenly sized as possible. Take a ziploc freezer bag and drip a few drops of water in it- this will keep the chicken from sticking to the bag as you pound it. Put one piece of chicken in the bag at a time and do not seal. I use a heavy pan to evenly and gradually pound the chicken until it is ~ 1/4 inch thick.
After all the chicken is pounded, sprinkle with salt, pepper, and sage. Sage is a pretty strong herb, so use as much or as little as you like. If you are using fresh sage, just place one leaf on each piece of chicken, and then a piece of prosciutto on top. Otherwise, just put a piece of prosciutto on top of each piece.
Once all the chicken is prepped, heat a very large skillet over medium-high heat with whatever fat you want to cook with. I used olive oil. Once the pan gets really hot place a few pieces of chicken in the pan prosciutto side down. It is very important that the pan is very hot so the prosciutto will sear onto the chicken. Don't move the chicken for a couple of minutes, then flip and sear on the other side for about 2 minutes. The chicken is thin, so it should cook pretty quickly. You will have to do this in batches, I did 3 pieces of chicken at a time.
When all the chicken is cooked and removed to a plate, add more oil if needed, and throw in the shallots and garlic. Sautee for about 30 seconds, then add the mushrooms. We got some oyster mushrooms from an asian market (only $3 for 8 0z instead of at least double that price at a grocery store), but you could use whatever you'd like- white button, cremini. Cook until soft then, with the pan still very hot, pour in the chicken broth to get all the delicious bits off the bottom of the pan. Let the liquid come up to a boil and simmer for 5 minutes. It will start to thicken up. Add a squeeze of lemon right at the end, and the capers. At this point you can serve the sauce right over the chicken, or add the chicken back into the pan to reheat.