Preheat oven to 190ºC . Place tortilla on a small baking sheet. Bake for 5 minutes, or until tortilla begins to crisp. Spread sauce to 1 cm of edge. Sprinkle tunan mushrooms, and tomato over tortilla. Arrange bell pepper half-rings on top. Sprinkle cheese over pizza. Bake for 8 to 10 minutes more or until heated through.