Butter 4 individual, shallow, flame-proof casseroles and set 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. (Recipe may be made ahead to this point several hours ahead; cover and chill.)
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.
Sprinkle the chopped mint over the vegetables and lay the mint sprigs on the salmon. Put the casseroles on serving plates and serve immediately.
Remove the gills and eyes from the fish, or have your fish store do it. Cut the heads and bones across into 4-inch pieces. Put them in a shallow pan and cover with cold water: Let stand for 1 hour, changing the water twice. Drain.
Heat the oil in a large pot over medium heat. Add the onion, fennel, leek, peppercorns, salt, parsley and bay leaf. Turn the heat to medium-low and cook until the vegetables are softened but not browned, about 4 minutes. Add the fish bones and cook, stirring from time to time, until the bones and any flesh around the bones turn from translucent to white, about 12 minutes. Add the wine and water and bring to a boil. Boil for 10 minutes, skimming off the foam as it rises to the top. Remove from the heat and let rest for 10 minutes.
Strain the fumet through a fine-mesh sieve, pressing firmly on the solids to extract as much of the flavorful liquid as possible. If you have more than 3 cups of fumet, place the liquid in a clean saucepan and boil until reduced to 3 cups. Store, tightly covered, in the refrigerator for up to 3 days, or in the freezer for up to 2 months.