Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions until al dente. Drain and set aside.

Reserve 1/2 cup of pasta water for later use.

In a skillet, heat olive oil over medium heat. Add the sliced leeks and cook for about 10 minutes until softened and caramelized, stirring occasionally.

Add the sliced mushrooms and minced garlic to the skillet. Cook for another 5-7 minutes until the mushrooms are browned and tender.

Reduce heat to low and stir in the heavy cream, thyme, salt, black pepper, and red pepper flakes. Allow to simmer for 5 minutes.

Gradually add the Gruyere cheese, stirring until melted and smooth. If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.

Add the cooked pasta to the skillet and toss to coat in the sauce. Cook for an additional 2 minutes to heat through.

Serve immediately, topped with extra Gruyere cheese and fresh thyme if desired.