Melt butter in a large nonstick skillet over medium-high heat. Add scallops; cook 3 minutes on each side or until done. Remove from pan, and keep warm. Add onion and bell pepper to pan; sauté 5 minutes or until tender.
Combine chicken broth, half-and-half, sour cream, flour, salt, paprika, black pepper, and nutmeg in a medium bowl; stir well with a whisk. Add broth mixture to pan; bring to a boil. Reduce heat to medium-low; simmer 5 minutes or until thick, stirring constantly. Remove from heat; stir in scallops, sherry, and diced pimiento.
Place English muffins on a foil-lined jelly-roll pan. Spoon about 1/2 cup scallop mixture over each muffin half. Sprinkle each muffin half with 1 1/2 teaspoons breadcrumbs and about 3/4 teaspoon Parmesan cheese. Broil 5 minutes or until browned and bubbly. Serve immediately.