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

In a skillet, heat the olive oil and butter over medium heat. Add the sliced leeks and cook for about 5 minutes until they start to soften.

Add the sliced mushrooms and continue to cook for another 10-15 minutes, stirring occasionally, until the leeks are caramelized and the mushrooms are browned.

Add the minced garlic to the skillet and cook for 1 minute until fragrant. Pour in the heavy cream and bring to a simmer.

Stir in the thyme, salt, and pepper. Let it simmer for 5 minutes until slightly thickened.

Add the cooked pasta to the skillet and toss to coat in the sauce. Remove from heat.

Preheat your oven's broiler. Transfer the pasta mixture to a baking dish and sprinkle the shredded Gruyere cheese on top.

Broil for about 3-5 minutes until the cheese is bubbly and golden. Serve hot.