Melt butter in a 12-inch nonstick skillet over medium-high heat. Add onions and garlic; sauté 1 minute or until tender. Add shrimp and scallops; sauté 3 minutes or until done. Reduce heat to medium-low.
Add half-and-half, salt, pepper, and crabmeat; cook 3 minutes or until thoroughly heated, stirring constantly (do not boil). Gradually sprinkle 1/2 cup cheese over seafood mixture, stirring constantly; cook 1 minute, stirring constantly. Remove from heat. Combine pasta and seafood mixture in a large bowl. Top each serving with 1 1/2 teaspoons cheese and 1 1/2 teaspoons parsley.
The sauce is like a traditional alfredo, with no flour or other thickener. Don't worry if it looks thin. It's the perfect consistency for coating the pasta. Pat the shrimp and scallops dry before cooking with paper towels so they don't dilute the sauce.