Preheat the oven to 350F. Pour the oil into a large oval flameproof casserole. Layer the outer lettuce leaves on the base and sides of the casserole. Sprinkle half the onions, tomatoes and lemons over the lettuce. Sprinkle with the thyme and parsley. Oil the tuna and place it in the casserole. Cover the tuna with the remaining lemon slices, tomatoes, and onions. Pour in the wine and Chartreuse. Bake uncovered, for 1 1/2 hours.
