Heat olive oil in a large heavy-bottomed pot or Dutch oven over medium heat, then add the diced onion and cook until it becomes tender, about 4 minutes.
Add the minced garlic and sauté for an additional 30 seconds until fragrant.
Pour in the marinara sauce, chicken broth, and sprinkle in the Italian seasoning; bring the mixture to a rolling boil.
Stir in the penne pasta, then reduce the heat to low and let it simmer gently; stir regularly to prevent the pasta from sticking to the pot.
Allow the pasta to cook uncovered for around 20 minutes, or until the pasta is tender and the sauce has thickened and reduced by half.
Remove the pot from the heat and fold in the shredded mozzarella and Parmesan cheeses, stirring until the cheese is melted and everything is well combined.
Serve your cheesy one pot pasta right away for the best flavor and texture.
