Cook pasta according to package directions. In a large skillet over medium heat, melt 6 tbsp butter. Mix in garlic and parsley; cook for a couple minutes until fragrant, then add asparagus and onion, season with salt and pepper; cook, stirring occasionally until asparagus is tender, about 5 minutes. Place scallops in pan and cook briefly on each side until lightly browned and the center is cooked through, about 3 minutes per side. Remove from heat when scallops are just finished. Meanwhile, in a small saucepan, combine milk, mascarpone and remaining 2 tbsp butter. Cook over medium heat, stirring until sauce is warm and butter is completely melted. Stir sauce and scallop mixture together, toss with pasta and serve immediately. Garnish with additional parsley, if desired.