In a steamer insert on top of boiling water, steam the lettuce leaves for 5 minutes. Drain in a colander.
In a medium saucepan, sauté the onion and leek slices over medium heat in 1 tbsp olive oil for about 15 minutes of until soft and golden. When ready, remove from heat and leave in warm spot.
In a heavy sauté pan or saucepan, sauté the cubed sweet potato pieces in 2 tbsp of olive oil over medium-high heat until soft and slightly browned on all sides. Add the mushroom slices and zucchini pieces and sauté a few minutes more until the mushrooms are soft and brown, mixing with wooden spoon regularly. Add back the onion mixture. Season to taste the vegetable mixture and remove from heat.
In a small bowl, whisk 1 egg, add 2 round tbsp of ricotta cheese, 1/2 cup light cream, 1/2 cup plain yogurt and 1/4 cup finely grated gruyere cheese. Salt and pepper and add 1/4 tsp chili pepper (or to taste).
In a buttered au-gratin (oven proof) dish, place half the steamed lettuce leaves, patted dry, to line bottom (not sides). Spread the vegetable mixture evenly over the lettuce leaves. Pour the egg-cheese mixture on top of the vegetables. Top with the rest of the lettuce leaves. Sprinkle with grated parmesan cheese to taste and dot with small butter squares.
Bake for 30 minutes in a 350 F oven or until set. Serves 4.