Heat 1 tablespoon olive oil in a large skillet or saucepan over medium-high heat. Add 1 yellow onion (diced) and cook, stirring, for 3 to 4 minutes or until softened. Add 3 garlic cloves (minced) and cook for 1 minute, stirring constantly, so it doesn't burn.
Add 1 pound penne pasta, 1½ cups marinara sauce, and 2½ cups chicken broth to the pan. Stir to combine, then bring to a simmer. Cover with a lid and cook, stirring occasionally, for about 8 minutes or until the pasta is al dente. Add extra water during the cooking process if looking a little dry.
Add 3 cups baby spinach leaves, ½ cup heavy cream, and ½ cup shredded mozzarella. Stir until the spinach has wilted and the mozzarella has melted. Season to taste with salt and ground black pepper.
Add in ¼ cup chopped basil and ¼ cup grated parmesan and serve.