Heat a medium skillet over medium heat. Once heated, add the chorizo. Cook, stirring and breaking up the meat occasionally, until cooked through, about 5-7 minutes. Push to one side of the pan and pour in the beaten egg. Cook the egg, scrambling and stirring with the chorizo, until firm, about 2-3 minutes. Remove from heat and let cool slightly.
In a small bowl, mix together the diced onion and chopped cilantro.
Assemble the tostadas: Spread a thin layer of refried beans on each corn cracker. Top with a spoonful of the chorizo and egg mixture. Top with the onion and cilantro and enjoy!