Heat broiler, with rack in highest position. In a medium bowl,
combine tuna, mayonnaise, onion, capers, lemon juice, salt, pepper,
and oregano and stir until thoroughly combined.
Arrange bread on a baking sheet and spread a little mayonnaise on
each slice. Divide tuna salad evenly among slices, then top each
with 2 slices tomato and 1 slice cheese. Broil until cheese is golden
brown and bubbling, 3 to 4 minutes.
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