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Adjust rack to middle position and preheat oven to 425 degrees. Line a baking sheet with foil and coat with nonstick cooking spray. Wash and dry produce.
Halve, peel, and very thinly slice onion; mince a few slices until you have 2 TBSP (4 TBSP for 4 servings). Drain and rinse beans. Dice tomato into ΒΌ-inch pieces.