in a medium skillet, fry the bacon until it is cooked through and very lightly browned, ~5 min. remove the bacon and set aside. pour out the excess bacon grease and return the skillet to the stove.
add the shallot and walnuts, toasting and shaking the pan, until the walnuts are lightly browned, 3-4 min. remove the pan from the heat and stir in the bacon and sage leaves.
season the fish inside and out with salt and pepper. brush the skin lightly with vegetable oil. divide the bacon mixture between the 4 trout, stuffing it in the cavity of each fish. place the fish in a large roasting pan fitted with a rack. roast until the fish is just cooked through, ~15 min.
transfer to plates and serve with lemon and garnished with the watercress.