Preheat oven to 350° F.
Cook manicotti according to package directions. Drain and set aside.
Meanwhile, dice mushroom, onion, and carrot; grate zucchini, crush garlic, and roughly chop spinach.
In a small bowl, mix Parmesan, mozzarella, and ricotta cheeses together with the garlic and egg.
Add the mushrooms, onion, zucchini, and spinach to the cheese mixture and stir until well combined.
Scoop mixture into a large zip-lock bag; seal, then cut off one corner. Squeeze filling into each cooked manicotti shell.
Place each stuffed shell into a large baking dish, top with sauce, then bake for 25-30 minutes.
Top with additional mozzarella if desired, and bake for an additional 5 minutes until cheese is melted and bubbly.
Meanwhile, add lettuce, tomatoes, and carrots to a large bowl; top with your favorite dressing and toss to combine.
Serve pasta with salad on the side.