1. Remove crusts from bread; spread one side lightly with butter. Cut slices into quarters and place buttered-side-down on tray or wax paper.
2. Microwave cream cheese in small dish about 30 seconds, on HIGH, or until softened. Stir in onion and horseradish.
3. Divide cream cheese mixture among bread squares; spread mixture to cover bread.
4. Preheat microwave browning grill for 5 minutes on HIGH.
5. urn tomatoes stem-side-down. Cut almost through each tomato twice to make petals. Spread apart tomato petals while arranging on each bread square. Place 1 medium or 1/2 large oyster in the center of each tomato. Arrange canapes on preheated grill.
6. Microwave for 1 to 1 1/2 minutes, on HIGH, or until bread is toasted.