Place everything into your pot: the spaghetti, sliced onion, garlic, mushrooms, tomatoes, spinach, basil, salt, pepper and crushed red pepper flakes. Add in the olive oil and then add in 4 1/2 cups of water. Bring everything to a high boil and turn it down just slightly after the water starts boiling if you need to. Then, you are literally just mixing everything up together using tongs.