Boil salted water in a large pot. Cook tagliatelle or fettuccine according to package instructions until al dente, about 8-10 minutes. Reserve ½ cup of pasta water before draining.

In a wide skillet, heat olive oil over medium heat. Add sliced leeks with a pinch of salt. Caramelize for 10-12 minutes until golden and soft.

Add butter to the skillet. Once melted, add sliced mushrooms and sauté for 5-7 minutes until golden brown. Mix in minced garlic and fresh thyme in the last minute.

Pour in vegetable broth, scraping the bottom to release browned bits. Simmer for about 2 minutes.

Reduce heat to low, add heavy cream, and stir. Let thicken for 3 minutes. Gradually mix in Gruyere cheese until melted and smooth.

Adjust the consistency with reserved pasta water, seasoning with salt, black pepper, and lemon juice to taste.

Add drained pasta to the skillet, tossing to coat in the sauce. Cook for another minute, then serve immediately.