Heat a large pot over medium heat and add the sliced smoked sausage; cook 2–3 minutes to brown slightly.
Add the diced onion, minced garlic, and sliced mushrooms to the pot; 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.
Cover the pot and cook for 5–8 minutes, then remove the lid and stir; continue cooking until the penne is al dente and most liquid is absorbed, about 3–5 more minutes as needed.
Reduce heat to low, stir in the Monterey Jack and Parmesan cheeses until melted, then fold in the fresh spinach until wilted and everything is well combined.
Adjust seasoning if desired and serve warm.