Preheat the oven to 230 degrees C. Slice the tomatoes into quarters; if your tomatoes are large, cut the quarters in half.
Heat the oil in a frying pan over a moderate heat. Add the onion and sauté for 5 minutes until soft. Stir in the tomatoes, then remove the pan from the heat.
Arrange the corn chips in single layers on two baking trays. Top the chips evenly with the vegetable mixture and spring onions. Sprinkle with the cheese.
Bake for 4–5 minutes, just until the cheese melts. Serve immediately, hot from the oven.