In a large pot of salted boiling water, cook penne according to package directions until al dente. Drain, reserving 1 cup pasta water, and return to pot.
Meanwhile, in a large skillet over medium heat, heat oil. Add onion and garlic and cook until soft, 5 minutes, then add spinach and roasted red peppers. Cook until wilted and warmed through, 3 minutes.
Stir in heavy cream, chicken broth, and 1/4 cup pasta water and bring to a simmer.
Add cooked penne to pan and stir until coated, then season with crushed red pepper flakes.