1. Butter the casseroles and set them aside. Trim and discard the thin belly section from the salmon fillets and season the fillets on both sides with salt and pepper. Place the salmon in the center of the casseroles and surround the fillets, in this order, with the peas, fava beans, snow peas, bacon, scallions, and lettuce. Season the vegetables with salt and pepper. Lay the butter slices over the vegetables. (The recipe can be made to this point up to several hours ahead; cover and refrigerate.)

2. Pour the fish fumet into the dishes and place each dish on top of a stove burner turned to high heat. Bring the fumet to a boil. Lower the heat to medium-high and cook for 5 minutes, until a metal skewer inserted into the center of the salmon for 5 seconds feels barely warm when touched to your lip; the salmon will be rare.

3. Sprinkle the chopped mint over the vegetables and lay the mint sprigs on the salmon. Put the casseroles on serving plates and serve immediately.