In a large pot or Dutch oven, heat a splash of oil over medium heat. Add the sliced smoked sausage and cook until browned, about 5-7 minutes.
Once the sausage is browned, add the diced onion and minced garlic to the pot. Sauté for another 2-3 minutes until the onion becomes translucent and fragrant.
Stir in the sliced mushrooms and cook for an additional 3-4 minutes, allowing them to soften.
Next, add the uncooked penne pasta, chicken stock, and undrained roasted red peppers to the pot. Stir everything together, ensuring the pasta is submerged in the liquid.
Bring the mixture to a boil, then reduce the heat to a simmer. Cover the pot and let it cook for about 12-15 minutes or until the pasta is al dente, stirring occasionally.
Once the pasta is cooked, remove the pot from heat and stir in the Monterey Jack cheese, Parmesan cheese, and fresh spinach.
Give the pasta a good stir to combine everything, then serve it hot. Enjoy this comforting One Pot Penne Pasta Recipe with your loved ones!