Preheat oven to 450 degrees. Divide eggplant, onion, tomatoes, and 2 tablespoons oil between two rimmed baking sheets. Season with salt and pepper; toss to combine. Roast until eggplant is tender, 25 to 30 minutes; let cool to room temperature.
Meanwhile, place bread on another baking sheet, and brush with 1 tablespoon oil. Sprinkle with salt and pepper; bake until golden, 4 to 6 minutes.
Transfer eggplant mixture to a medium bowl. Add mint; toss with remaining tablespoon oil and lemon juice. Divide eggplant mixture among bread slices, top with feta, and serve.