Preheat oven to 400°.
Combine beans, bell pepper, 1/2 cup mint, broth, garlic, red potatoes, and onions; toss well. Arrange vegetable mixture in a single layer on a jelly-roll pan coated with cooking spray; cover with foil. Bake at 400° for 20 minutes. Uncover and stir; bake, uncovered, an additional 40 minutes or until lightly browned, stirring after 20 minutes. Let stand 5 minutes. Place vegetable mixture in a large bowl.
Combine remaining 1/2 cup of mint, vinegar, oil, salt, and black pepper, stirring well with a whisk. Drizzle vinaigrette over vegetable mixture, and toss well to coat. Serve the salad at room temperature or chilled.