In a medium nonstick skillet over medium heat, add ½ tablespoon vegetable oil. Add chorizo and break into small pieces using a wooden spatula. Cook until cook through and the color is slightly darker, about 4 minutes. Transfer to a plate lined with paper towels.
Taste to check the saltiness of the chorizo. If the chorizo is very salty, do not add salt into the egg. If the chorizo is seasoned just right, season the egg lightly.
In the same nonstick skillet over medium-low heat, wipe off most of the grease. Add ½ tablespoon vegetable oil. Add onion and saute for 3 minutes. Add tomato and saute until soften, about 4 minutes. Pour in the eggs and gently stir to make scrambled eggs. Cook until just set. Do not overcook. Transfer to a serving plate.
Meanwhile, start on the tortillas. There are 2 ways to reheat them. For open flame, place over the open flame on the stove over medium heat until slightly brown, but still soft on both sides. Watch carefully because the tortillas can be burnt easily. For microwave, stack 6 tortillas and wrap with of a paper towel. Add a few drops of water on the paper towel. Warm in the microwave for about 35 seconds or until hot. Repeat with the other tortillas. Transfer the hot tortillas to a tortilla warmer or a plate with an aluminum-foil-cover.
It's best to let everyone assemble their own tacos when serving. Place scrambled egg on a tortilla. Sprinkle cotija crumbles over if using. Serve immediately.
Notes
I got my chorizo from a Latino supermarket, but you can always find chorizo in regular supermarket. Mine is not spicy. I only used a little bit and freeze the rest.