Mix the tuna, onion, scallion, parsley, mayonnaise, and a few drops of tabasco in a bowl. Season with salt and pepper. Set aside.
To prepare the ketchup: Heat the olive oil in a saucepan over medium-high heat and sauté the garlic and onion until soft but not browned, about 2 to 3 minutes, stirring frequently. Add the peeled tomatoes, tomato purée, suga