In a 9- to 10-inch cast-iron or other ovenproof skillet, heat the olive oil over medium heat.
Add the diced onion and bell pepper. Sauté for 5-6 minutes or until softened.
Add the chorizo, breaking up the pieces with a wooden spoon. Cook for about 5 more minutes, and then turn off the heat.
While the chorizo cooks, in a large bowl, whisk together the eggs, queso fresco, milk, cilantro, salt, and black pepper.
Pour the egg mixture over the chorizo, and give everything a stir.
Transfer to the oven and cook 14-16 minutes, just until the eggs are set. (I like to remove the frittata from the oven just shy of them being fully cooked, because they’ll continue to cook slightly even when they come out of the oven.)
Remove from the oven and let sit for 5 minutes. Slice and top with more crumbled queso fresco and chopped cilantro.