Bring a large pot of salted water to a boil over high heat. Add pasta and cook for 8-10 minutes until al dente, reserving ½ cup of pasta water. Drain and set aside.

In a large skillet, heat olive oil over medium heat. Add sliced leeks and a pinch of salt, sauté for 10-12 minutes until golden brown.

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

Pour in vegetable broth, scraping the skillet's bottom. Let simmer for 2-3 minutes before reducing heat to low.

Stir in heavy cream and simmer for 3-4 minutes until thickened.

Add Gruyere cheese to the sauce, stirring until fully melted. Adjust consistency with reserved pasta water if needed.

Combine the cooked pasta with the sauce, seasoning to taste with salt and pepper, and add lemon juice to brighten the dish.