In a large pot of salted water, boil pasta until al dente, according to the package instructions.
While the pasta is boiling, melt butter in a saute pan set over medium high heat, then add sliced kielbasa, onion, and bell pepper. Saute until the vegetables are tender and the sausage is seared. Then stir in the fresh spinach and cook until wilted. Remove all ingredients from the pan with a slotted spoon and set aside, leaving the extra butter in the pan.
Reduce the heat to medium low and sprinkle in the flour. Whisk the flour into the butter and continue whisking while cooking for two minutes. Then slowly add the chicken broth while continuing to whisk. Next, whisk in the heavy cream and mustard.
Add the Parmesan cheese to the sauce and whisk until completely melted and smooth, then add seasonings. Bring the sauce to a simmer and cook until it has thickened.
Strain the cooked pasta from the water and add the pasta to the sauce along with the cooked kielbasa mixture that was set aside. Toss everything together until well combined, then serve.