Fill a large stock pot with water and place on the stove on high. While you're waiting for this to come to a boil, clean and slice the leeks, brussels spouts and mushrooms. Heat a large skillet over medium/medium high heat on the stove. Add the butter & olive oil. When melted, add the vegetables and season with salt, pepper & red pepper flakes. Stirring occasionally.
When the pasta water is boiling, season with about a tablespoon of salt and add the pasta.
Back to the sauce. Add the butternut soup to the vegetables and the cream. Stir together well. When the pasta is done, reserve 1 cup pasta water and drain the pasta. Immediately add the pasta to the sauce. Sprinkle with the cheese and mix together really well.
If the sauce gets too "stiff" add the reserved pasta water one ladle full at a time until it loosens up.