Preheat oven to 350 degrees Fahrenheit.

Heat oil in a large skillet, then sauté onion until translucent. Add mushrooms and cook for approximately 10 minutes. Add zucchini and cook until tender.

Add tomato sauce, then fill one empty tomato sauce can with water and add to sauce. Season with salt and pepper to taste. Cook over medium heat until bubbly.

Combine farmer cheese, mozzarella cheese, eggs, salt, and pepper. Stuff manicotti shells with the filling using a spoon or piping bag and place into a 9×13 baking dish.

Pour sauce over manicotti shells. Cover and cook for 45 minutes or until pasta shells are soft.

Uncover, sprinkle with mozzarella cheese, and cook until cheese is melted.