Bring a large pot of salted water to a boil. Cook pasta to al dente. Reserve 1 cup of water before draining.

While water heats, melt 2 tbsp butter in a large skillet over medium-high heat. Add mushrooms and sear without stirring for 3-4 mins until golden. Flip and cook until tender. Remove and set aside.

In the same pan, melt remaining 2 tbsp butter over medium heat. Add cleaned leeks and cook for 10-15 minutes until soft and caramelized.

Add minced garlic and thyme to the leeks; cook for 1 minute.

Deglaze pan with white wine, scraping up browned bits. Simmer until reduced by half.

Lower heat. Stir in heavy cream and simmer gently. Gradually whisk in grated Gruyère until smooth.

Return mushrooms and cooked pasta to the pan. Toss vigorously to coat, adding splashes of reserved pasta water to reach desired consistency.

Serve immediately topped with parsley and extra cheese.