Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add onion, and saute 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. Yield: 8 servings (serving size: 3 ounces fish and 2 tablespoons onion mixture). Per serving: 316 Calories; 11g Fat (33% calories from fat); 41g Protein; 10g Carbohydrate; 65mg Cholesterol; 134mg Sodium Serving Ideas : Garnish with mint sprigs, if desired. NOTES : Serve tuna with onion mixture. Recipe by: Cooking Light, May 1995, page 81 Posted to MC-Recipe Digest V1 #411 by igor@digex.net on Jan 28, 1997.