In a large skillet, melt the butter over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant, being careful not to burn it.
Add the shrimp to the skillet and cook for 2-3 minutes until they turn pink. Remove the shrimp from the skillet and set aside.
In the same skillet, add the cleaned mussels and chicken broth. Cover and cook for about 5 minutes until the mussels open up. Discard any that remain closed.
Reduce the heat to low, then stir in the heavy cream, lemon juice, and red pepper flakes (if using). Season with salt and pepper to taste.
Return the cooked shrimp to the skillet and stir to combine. Cook for an additional 2-3 minutes until heated through.
Serve immediately over cooked pasta or with crusty bread, garnished with fresh parsley.