Add the oil to the skillet, along with the onion and chopped bell peppers. Cook until the onion is lightly browned and the peppers have softened up (about 7-8 minutes).
Reduce the heat to medium. Add the butter and garlic to the skillet, and once the butter melts, sprinkle in the flour and cook for about a minute, stirring well. Stir in the cream and Dijon mustard. The sauce should thicken quite quickly, so don't let it cook for more than a minute or so before moving on to the next step.

Add the cooked sausage and spinach to the skillet and stir/toss.
Before draining the pasta, add in a splash (a couple tablespoons or so) of the hot pasta water to the skillet, then drain the pasta and add it to the skillet. Toss until the spinach has wilted and the pasta is coated with the sauce. Serve immediately with salt & pepper as needed and some freshly grated parmesan cheese.