Boil a salted pot of water for the pasta and cook it al dente according to package directions.
Meanwhile, add the drained roasted red peppers, garlic, Italian seasoning, and olive oil to a food processor and blend until smooth.
To a skillet, melt the butter over medium heat and cook the onion for 4-5 minutes or until softened and lightly browned.
Add the roasted red pepper mixture, cream, and Dijon mustard to the skillet and cook until it's thickened up a bit (about 7-8 minutes), stirring occasionally. This is not meant to be a super thick sauce. Give it a taste and season with salt & pepper (I am generous with both).
Stir in the spinach and basil, then add in the drained pasta and toss until the spinach has wilted (add a splash of the hot pasta water prior to draining the pasta if desired, especially if you want to thin the sauce more).
Serve immediately with freshly grated parmesan cheese.