Seared Tuna Sandwich with Balsamic Onions

Ingredients

• 4 (2-ounce) whole wheat hamburger buns
• 1 ½ tablespoons balsamic vinegar
• 2 cups vertically sliced onion
• 2 teaspoons butter
• ¼ teaspoon black pepper
• ¼ teaspoon garlic powder
• ½ teaspoon salt
• + 7 more ingredients
• • 1 tablespoon all-purpose flour
  • 4 curly leaf lettuce leaves
  • 4 (1/4-inch-thick) slices tomato
  • 4 teaspoons Dijon mustard
  • Cooking spray
  • 4 (6-ounce) tuna steaks (about1/2 inch thick)
  • ¼ teaspoon dried thyme

Melt butter in a medium skillet over medium-high heat. Add onion; sauté 5 minutes. Stir in vinegar and thyme; cover and cook 5 minutes, stirring occasionally. While onion cooks, combine flour, salt, garlic powder, and pepper in a shallow dish. Dredge fish in flour mixture. Heat a large nonstick s...

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