In a large skillet with a lid, melt the butter over medium heat. Add the finely chopped onion, crushed garlic, and chili flakes. Sauté until the onion softens and becomes translucent, releasing its sweet aroma.

Pour in the dry white wine and let it simmer until it reduces by two-thirds, concentrating its flavors. Stir in the fresh cream and allow it to simmer gently for 2-3 minutes, creating a rich, velvety base.

Add the mussels to the skillet, ensuring they are evenly distributed. Cover the skillet with its lid and lower the heat. Steam the mussels gently for 8-10 minutes, or until they are fully cooked and their shells have opened.

Remove the skillet from the heat. Season the dish with salt and freshly ground black pepper to taste. Stir in the freshly squeezed lemon juice, finely grated zest, and chopped parsley for a burst of brightness and freshness.

Serve the mussels immediately, ensuring each portion is generously coated with the flavorful sauce. Enjoy this dish with crusty bread or over a bed of pasta to savor every drop of the delicious lemon garlic butter sauce.