Set oven to 250 °. Peel and finely chop onions and garlic.
Fry the onion in 2 tablespoons oil for about 2 minutes. Add the squash, cook for about 2 minutes. Pour over crushed tomatoes. Add tomato puree and broth cubes.
Season with 1 teaspoon salt and 1/2 teaspoon pepper. Let simmer without lid about 15 min.
Peel and cut the sweet potato into smaller pieces. Boil it soft in lightly salted water.
Let the water drain and crush the potatoes roughly. Season with 1 teaspoon salt.
Pour the Quorn sauce into an ovenproof mold and top with the potato. Sprinkle with cheese.
Grate in the middle of the oven for about 10 minutes. Serve directly with lettuce.