Put a large pot of salted water over high heat to bring to a boil.
Meanwhile, add olive oil, butter, leeks, and a generous pinch of salt and pepper to a pan and sautée over medium heat until they have softened and become a bit translucent. Add mushrooms and cook until they have softened, but still have a bit of bite.
While leeks and mushrooms are cooking, boil tortellini. If the pasta is fresh, this will only take 3-4 minutes. Drain, reserving about a cup of pasta water.
Combine tortellini, mushrooms, leeks, crème fraîche, and a bit of pasta water until you have a very thin sauce covering the entire dish.
If you are using arugula, add at this time. The arugula will just slightly wilt.
Taste, and add salt and pepper if needed.
Plate the pasta, adding whatever amount of parmesan makes your heart sing.