Bring a large pot of salted water to a rolling boil and add the pasta. Cook until just shy of al dente (about 1 minute less than package instructions). Reserve 1 cup of pasta cooking water, then drain the pasta.

Prepare the leeks: trim dark green tops, slice the white/light-green portion lengthwise, then into thin half-moons. Rinse thoroughly in a bowl of cold water to remove trapped dirt. Pat dry.

Heat a large skillet over medium-high heat. Add olive oil and 1 tablespoon of butter. When the butter foams, add the leeks and sauté for 3–4 minutes until softened.

Add the sliced mushrooms. Spread them in an even layer and let sit without stirring for 2–3 minutes. Then stir and season with a pinch of salt and pepper. Continue cooking for 4–5 minutes until browned.

Stir in minced garlic and cook for 30 seconds until fragrant.

Lower heat to medium-low. Add heavy cream and the remaining tablespoon of butter. Let the cream warm and thicken for 1–2 minutes without boiling.

Remove the pan from the heat and add the shredded Gruyère in handfuls, stirring gently until it melts into a smooth sauce. Adjust thickness with reserved pasta water if necessary.

Add the drained pasta to the skillet and toss to coat, adjusting sauce consistency as needed with more reserved pasta water. Season with more salt and pepper to taste.

Serve immediately, garnished with chopped parsley and additional Gruyère or cracked black pepper.