Heat a large pot over medium heat and add the sliced smoked sausage; cook until lightly browned, about 3 minutes.
Add the diced onion, minced garlic, and sliced mushrooms to the pot and cook, stirring often, until the onion is slightly tender, about 3–4 minutes.
Stir in the uncooked penne, chicken stock, and the jarred roasted red peppers with their liquid; mix to combine and bring to a gentle simmer.
Cover the pot and cook for 5–8 minutes, then remove the lid and stir; continue cooking until the penne is al dente, about 2–4 more minutes, stirring occasionally.
Stir in the shredded Monterey Jack and grated Parmesan until melted and combined, then add the spinach and stir until wilted and evenly incorporated.
Adjust seasoning if desired, then serve hot.