Preheat oven to 375°F.
Slice bread into 1-inch rounds and lightly butter each slice. Sprinkle with a touch of garlic powder.
Toast in the oven for about 1 minute, just until slightly crisp on top (don’t overdo it — they’ll bake again later).
Mix the topping: In a bowl, combine mayonnaise, chopped black olives, shredded cheddar, diced red onion, pepper, garlic powder, and a pinch of salt.
Spread generously over each toasted slice — about 1 to 1½ tablespoons per crostini, edge to edge.
Broil for a few minutes until the tops are bubbly and golden brown.
Serve immediately and watch them disappear!