Preheat oven to 425°.
Combine corn and 1 teaspoon oil in a jelly-roll pan coated with cooking spray. Bake at 425° for 20 minutes or until browned, stirring occasionally.
Combine 2 teaspoons oil, vinegar, mustard, salt, and black pepper in a medium bowl; add corn mixture, stirring well. Stir in tomato, bell pepper, and onions. Serve warm or at room temperature.