Heat the olive oil over medium heat in a large saute pan. Add the onions, sugar, cinnamon and red pepper flakes and sprinkle with salt and pepper. Cook, stirring occasionally, until the onions are golden brown and caramelized, about 30 minutes. Stir in a few tablespoons of water and the vinegar. Taste and season with salt and pepper. Stir in the mint and chopped parsley just before serving.
Heat the grill to high for direct grilling. Brush the tuna on both sides with the canola oil and sprinkle with salt and pepper. Grill the tuna until golden brown and slightly charred on both sides and cooked to medium-rare doneness, about 2 minutes each side.
Slice the tuna steaks thickly across the grain and shingle them on a serving platter. Spoon the onion mixture down the center. Keep at room temperature about 15 minutes before serving.