In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package directions until al dente. Drain and set aside, reserving 1 cup of pasta water.

In the same pot (or a large skillet), heat olive oil and 1 tablespoon of butter over medium heat. Add sliced leeks and a pinch of salt, cooking until they start to soften and caramelize, about 5 minutes.

Stir in the sliced mushrooms and cook for an additional 5-7 minutes, or until they are tender and have released their moisture. Add the minced garlic and cook for another minute until fragrant.

Lower the heat and add the remaining butter. Stir to melt. Then, mix in the cooked pasta and a splash of reserved pasta water to help the sauce coat the pasta.

Gradually sprinkle in the shredded Gruyere cheese, stirring until the pasta is creamy and the cheese has melted. If the pasta appears too dry, add more reserved pasta water as needed.

Taste and adjust seasoning with salt and pepper. Serve hot, garnished with fresh parsley.