Heat a teaspoon of oil in a pan. Add the Tortilla. Cook on medium heat until the tortilla starts to puff up. Flip over and cook the other side. Slide the tortilla on a plate.
Heat another teaspoon of the oil in the same pan. Add the onions. Cook for 2-3 minutes, or until the onions just start to soften. Add the tomatoes. Cook for additional minute. Slide the mixture on top of your fried tortilla.
In the same pan, melt the butter. Add the eggs. Salt and pepper as desired. Cook sunny side up, until the egg whites set. To get the eggs to cook more evenly, cover the pan for a few seconds. Slide the eggs on your tortilla.
Top with a generous amount of Queso Fresco.