Finely chop the onion, cut the mushrooms into slices.
Fry the onions and mushrooms in vegetable oil.
Add mashed tomatoes and salt to taste. Mix well. Cook for 5-7 minutes over low heat.
Boil the spaghetti until tender in salted water. Throw in a colander.
Place the spaghetti in a skillet with the sauce and mix well.
Put on a platter with lettuce and serve.