Heat the garlic in the oil over low heat until it turns pale gold, about 15 minutes. Remove garlic with a slotted spoon to a small bowl.
Increase the heat to med-high and add the red onions. Cook for 2 minutes. Add the tuna strips. Increase the heat to high and quickly sear the fish on all sides, while seasoning it with the basil, oregano, and mint. Cook until the fish is done to your liking, 2 to 5 minutes. Remove fish to a hot platter or warm oven.
Return the garlic to the skillet. Add the tomatoes, olives, capers, and red pepper flakes. Cook for just a minute, stirring occasionally. Spoon over fish.