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

Drain the pasta and set aside, reserving about 1 cup of pasta water.

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.

Stir in the minced garlic, thyme, salt, and pepper, and cook for an additional 2 minutes.

Reduce the heat to low and pour in the heavy cream, stirring to combine. Allow it to simmer for 5 minutes.

Add the cooked pasta to the skillet, tossing to coat in the sauce. If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.

Stir in the shredded Gruyere cheese until melted and creamy.

Taste and adjust seasoning if necessary. Serve hot, garnished with additional thyme or cheese if desired.