Cook the pasta according to package instructions until al dente. Reserve 1 cup of pasta water, then drain and set aside.

Heat 2 tablespoon olive oil and 1 tablespoon butter in a large skillet over medium heat. Add the sliced leeks and cook for 10-12 minutes, stirring occasionally, until soft and golden brown. Season with a pinch of salt and pepper. Remove the caramelized leeks from the skillet and set aside.

In the same skillet, add the remaining 1 tablespoon olive oil and 1 tablespoon butter. Add the sliced mushrooms and cook for 5-7 minutes until browned and tender. Season with salt and pepper. Add the minced garlic and cook for an additional 1-2 minutes, until fragrant.

Add the white wine to the skillet, scraping up any browned bits from the bottom. Cook for 2-3 minutes until the wine is mostly evaporated. Stir in the heavy cream and bring to a gentle simmer. Cook for 3-4 minutes until slightly thickened.

Lower the heat and stir in the grated Gruyere cheese, mixing until melted and smooth. Add the cooked pasta to the skillet, tossing to coat. If the sauce is too thick, add reserved pasta water a little at a time until the desired consistency is reached.

Add the caramelized leeks back to the skillet, stirring to combine. Serve hot, garnished with chopped fresh parsley.