In a large pot, bring 2-2.5 quarts/liters of water to a boil. Add ½ tablespoon salt, followed by the pasta. Cook until al dente (i.e., slightly firm to the bite, not crunchy nor soft). Drain well.

As the pasta boils, make the sauce: In a large skillet (large enough to accommodate the pasta), melt 2 tablespoons butter over medium or medium-high heat. Add onion and sauté until tender, stirring occasionally. Add garlic and mushrooms, and cook until mushrooms are tender, about 4-5 minutes. Season with salt and pepper to taste.

Add the rest of the butter and heavy cream, and bring just to a boil. Reduce heat and let simmer for several minutes, until sauce has thickened slightly. Stir in nutmeg. Add ¼ cup parmesan, then more if desired. Adjust seasoning to taste. Stir in pasta and cook on very low heat, tossing to combine for 1 minute until warm.

Serve immediately. Garnish with parsley or chives if desired.