Preheat oven to 375 degrees. In a food processor, combine cheddar, bacon, and onion. Process until a smooth paste forms, and transfer to a small bowl.
Arrange bread on two rimmed baking sheets. Bake until crisp but not browned, about 6 minutes. Remove from oven and let cool completely.
Heat broiler. Top each toast with 2 tablespoons cheese mixture. Broil until topping is lightly browned, about 4 minutes.