Dice the onion and mince the garlic. Add both to a large skillet with the olive oil. Use a vegetable peeler or the side of a spoon to scrape the skin from the ginger. Once peeled, grate the ginger on a cheese grater straight into the skillet. Sauté the onion, garlic, and ginger over medium-low heat for about 5 minutes, or until the onion is soft and transparent. While these are cooking, dice the tomato.