Step 1

Heat oil in a large skillet over medium-high. Add mushrooms and cook, stirring, until browned, 6 to 7 minutes. Add leek; season with salt and pepper and cook, stirring, until just tender, about 3 minutes.

Step 2

Meanwhile, bring a large pot of salted water to a boil; add pasta and cook according to package directions until al dente. Reserve 1 cup pasta water; drain. Transfer pasta to skillet over medium heat; stir in creme fraiche, Parmigiano-Reggiano, and 1/2 cup pasta water until smooth, adding more pasta water as needed until creamy. Remove from heat; fold in arugula. Serve with more cheese.