How to Make It
Preheat the oven to 400°. Arrange the baguette slices on a baking sheet and brush generously with olive oil. Toast for about 10 minutes, until the bread is golden brown; let cool.
Meanwhile, in a small saucepan, boil the balsamic vinegar over moderately high heat until reduced to 1/4 cup, about 12 minutes; let cool.
Arrange the toasts on a platter and top with the sardines. Drizzle with a little of the reduced balsamic. Serve with flaky salt for sprinkling.