Add the dry pasta, cherry tomatoes, garlic, onion, zucchini, mushrooms, red pepper flakes, salt, marinara sauce, and water to a large pot with deep sides. Stir to combine.

Bring the pot to a boil over high heat, stirring once to separate the pasta.

Reduce the heat to medium-low. Cook for 10 to 14 minutes, stirring every 2 minutes to prevent sticking, until the pasta is al dente and most of the liquid has been absorbed.

Remove the pot from the heat. Fold in the fresh spinach and stir until it wilts, about 1 minute.

Taste and adjust the salt and red pepper flakes. Divide into bowls and serve immediately.