Place 1/4 cup oil and onion in skillet over low heat, 10 to 12 minutes. Add tomatoes, 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook over moderate heat, partially covered until thickened about 5 minutes. Remove from heat and set aside.
Season tuna with remaining salt and pepper.
In medium skillet, heat remaining oil. Add tuna and cook over medium heat about 4 minutes. Add cooked tuna, mint and garlic to tomato sauce and cook 2 to 3 minutes. Cook pasta, drain. Add sauce to pasta.