Cook Pasta: Cook the penne pasta in a large pot of salted boiling water according to package directions, or until al dente. Reserve about 1/2 cup of the starchy pasta water before you drain it.

Brown Sausage and Vegetables: Heat the olive oil in a large skillet over medium-high heat. Add the sausage, onions, garlic, red pepper flakes, and salt. Cook, stirring often, until the sausage is no longer pink and the onions are soft, about 5-7 minutes. Drain any excess fat if needed.

Add Cream: Reduce the heat to low and pour in the heavy cream. Bring the mixture to a gentle simmer, then add the baby spinach.

Wilt Spinach and Add Cheese: Stir continuously until the spinach has completely wilted, about 2-3 minutes. Mix in the shredded parmesan cheese, stirring until it’s completely melted and the sauce is smooth and creamy. Add reserved pasta water if the sauce seems too thick.

Combine and Serve: Add the cooked, drained pasta to the skillet with the sauce. Stir everything together until the pasta is coated in that dreamy, luscious sauce. Serve immediately.