Bring a large pot of salted water to a boil and cook the fresh tagliatelle or fettuccine until al dente. Reserve ½ cup of pasta water, then drain and set aside.

Heat olive oil in a large skillet over medium heat. Add chopped leeks and a pinch of salt, sautéing for 10-12 minutes until golden brown and soft.

Add butter and sliced mushrooms to the skillet, sautéing for 5-7 minutes until browned. Stir in minced garlic and thyme, cooking for an additional minute.

Pour vegetable broth into the skillet, scraping up the browned bits. Then add heavy cream and simmer for 3-5 minutes until slightly thickened.

Lower heat and gradually mix in grated Gruyere cheese, stirring until melted; adjust with reserved pasta water if needed.

Combine cooked pasta with the sauce, tossing gently to coat. Season with salt, pepper, and lemon juice before serving.