Boil penne pasta in a large pot of salted water per the package directions until al dente. Drain well, reserve the pasta water.
In a dutch oven or large saucepan, cook the Italian sausage and onion in a light drizzle of olive oil over medium-high heat. Cook until no pink remains, around 8 minutes, then drain excess grease from the pan.
Add the garlic, Italian seasoning, salt, black pepper and crushed red pepper flakes to the pan, cook for 1-2 minutes longer.
On medium-low, add the cream to the pot, simmer for 2-3 minutes. Add the spinach and sundried tomatoes. Cook while stirring, just until the spinach has wilted.
Add the cooked pasta and Parmesan cheese, stir to evenly coat. Add reserved pasta water in small amounts, to thin the sauce to your taste.
Add one tablespoon of basil pesto at a time, increasing the amount to your taste. Simmer just until heated through.
Serve immediately topped with additional Parmesan cheese.