Cook pasta according to package directions until al dente; reserve 1 cup pasta water, then drain.
Heat 2 tablespoons olive oil and 1 tablespoon butter in a large skillet over medium heat.
Add sliced leeks and cook 10-12 minutes, stirring occasionally until soft and caramelized; season with salt and pepper, then remove and set aside.
Add remaining olive oil and butter to the skillet; sauté mushrooms 5-7 minutes until browned and tender, seasoning with salt and pepper.
Add minced garlic and cook for 1-2 minutes until fragrant.
Pour in white wine, scraping browned bits; cook 2-3 minutes until mostly evaporated.
Stir in heavy cream and simmer gently for 3-4 minutes until slightly thickened.
Reduce heat; stir in grated Gruyere cheese until melted and smooth.
Add cooked pasta and toss to coat with sauce, adding reserved pasta water as needed for desired consistency.
Return caramelized leeks to skillet and stir to combine.
Serve immediately, garnished with fresh parsley.