There are two components you can prepare in advance; the mushrooms and the pearl onions. Let's start with the mushrooms. Just saute your sliced mushrooms in some butter and oil. They'll first absorb the liquids in the pan, and then release them. Once they release the liquids and start to brown, you can season with salt and pepper. Remove them from the heat, toss in some sliced green onions, and saute for a couple more minutes. Then you can just set them aside while you prepare the rest of the meal.
While your onions are browning, you can make your satchel of herbs. In a piece of cheesecloth, wrap up some parsley, thyme, and a bay leaf. Wrap it up and tie it with some twine.