Lightly drizzle each bread round with olive oil and top with equal amounts of garlic. Sprinkle lightly with salt. Split the sardines lengthwise (remove the bones if you wish) and top each bread round with an equal amount. Set the open sandwiches on a baking rack or oiled baking sheet and toast in a toaster oven for 1 minute or in a heated oven set to 350°F for 2 minutes. Allow the open faced sandwiches to cool slightly, and then top with the onion and tomato slices.