Bring a large pot of salted water to boil over high heat. Add the penne pasta and cook according to the package instructions until al dente. Reserve some pasta water for later

Meanwhile, melt some butter with olive oil in a large skillet. Add onion and saute until soft and fragrant, stirring occasionally so it doesn't burn. Add chopped garlic and saute until fragrant. Add baby spinach to the sauteed aromatics, season with Italian seasoning, salt, black pepper, and red pepper flakes, stir to combine, and cook until down until wilted.

Add heavy cream and Parmesan cheese, stir to combine, and bring to a gentle simmer for another 3-4 minutes until the sauce has slightly thickened and the flavor meld. Taste and adjust the salt accordingly. Bear in mind that the pasta is already salted and you may still need to add the pasta water.

Drain the cooked pasta and add it to the creamy spinach sauce. Stir or toss everything together until evenly coated with the sauce and serve immediately. Enjoy!