Preheat the oven to 375 degrees F. Heat a medium skillet over medium heat and add the oil. When the oil is hot add the onion, red pepper, chorizo, oregano, and garlic. Cook, stirring frequently, until the pepper and onions are soft and brown, about 6 minutes. Cool.
Whisk the eggs, milk, cilantro, and 1 teaspoon salt together in a large liquid measuring cup or medium bowl.
Arrange the tart shells on a baking sheet and divide the chorizo filling evenly among the tart shells. Pour or spoon the custard mixture over the filling to fill the shells. Bake until the eggs are just set, about 15 minutes. Cool and serve.