Preheat oven to 375 degrees.
Wash and dry your spaghetti squash, then pierce it with a fork several times on each side.
Place it in a baking dish along with 1 cup of water and bake for about 1 hour.
Heat marinara sauce and add chopped onion, pepper and mushrooms to the sauce.
Simmer until veggies have softened.
When your spaghetti squash has finished baking, let it cool a bit until holding it is comfortable, then slice it in half and remove seeds.
Pull a fork through the flesh from top to bottom, creating long threads.
Pile up your squash "noodles" and top with sauce.