Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add onion, and sauté 5 minutes or until lightly browned. Add vinegar, 1/8 teaspoon salt, and 1/8 teaspoon pepper, and cook for 2 minutes or until most of liquid evaporates. Remove the onion mixture from skillet; set aside, and keep warm. Wipe skillet clean with a paper towel.
Combine the flour and remaining salt and pepper in a shallow dish, and stir well. Dredge tuna in flour mixture. Heat 2 teaspoons oil in skillet over medium heat. Add tuna, and cook for 2 minutes on each side until tuna is medium-rare or desired degree of doneness. Serve tuna with onion mixture. Garnish with mint sprigs, if desired.