Heat a medium pot over medium heat and add the sliced smoked sausage. Cook, stirring occasionally, until the slices begin to brown, about 3–4 minutes.
Add the diced onion, minced garlic, and sliced mushrooms to the pot. Cook, stirring often, until the onion is slightly tender and the mushrooms soften, about 2–3 minutes.
Stir in the uncooked penne, chicken stock (or broth), 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 uncovered until the penne is al dente and the liquid is mostly absorbed, about 2–4 more minutes.
Stir in the Monterey Jack and Parmesan cheeses until melted and well combined, then add the spinach and cook until wilted, about 1–2 minutes.
Taste and adjust seasoning if desired, then serve immediately.
