Preheat your grill to medium-high heat.
In a small saucepan over medium heat, add 2 teaspoons of olive oil.
Heat the oil for 2-3 minutes, then stir in ground cinnamon and let it heat for 4-6 minutes.
Add the sliced red onion, red wine vinegar, sugar, and Kosher salt to a medium bowl.
Stir well to combine and set aside.
Brush the tuna steaks with olive oil and season them with salt and pepper to taste.
Place the tuna steaks on the grill and cook for 2-3 minutes on each side for medium-rare.
Divide the grilled tuna steaks among plates and top each steak with the caramelized onion mixture.
Sprinkle with chopped mint and parsley right before serving.