Melt butter in a small skillet over medium-high heat; add onion and garlic, and sauté until crisp-tender. Remove mixture from heat, and cool slightly.
Stir in sour cream, milk, and pepper. Toss with pasta. Sprinkle with cheese and parsley. Garnish, if desired. Makes 4 servings.
For Lemon Linguine, substitute 1 tablespoon fresh lemon juice for milk. Decrease pepper to 1/2 teaspoon, and add 1 tablespoon grated lemon rind.