Preheat the oven to 350°. Oil a baking sheet.
Lightly drizzle each slice of bread with olive oil and top with garlic. Sprinkle lightly with salt. Split the sardines lengthwise (remove the bones if you wish) and top each slice with an equal amount.
Set the sandwiches on the baking sheet and toast for 2 minutes. Let cool slightly. Serve topped with onion and tomato slices.