Par-boil potatoes until tender and drain. In a 5 quart stock pot, saute onions in butter together with the garlic, parsley, red pepper and green peppercorns until onions are clear. Drain clams, and reserve the juice. Add clams, crabmeat (cut in small chunks) and scallops. Simmer until scallops loose their glassy appearance. Add broth, clam juice, and potatoes, and bring to a boil. Combine remaining butter and the flour in a saucepan to make beurre manie. Remove a small amount of broth and whisk into the beurre manie. Add milk to the chowder and bring to a simmer. Whisk in the broth/butter/flour mixture a little at a time until desired consistency is reached. Add sherry and pepper to taste. Simmer for 5 more minutes then serve garnished with a sprinkle of cayenne and a sprig of parsley.