In a medium saucepan, bring 3 cups of water to a boil. Add the barley and return to a boil over medium heat. Lower the heat and simmer until barley is tender, but still chewy, about 40-50 minutes. Drain and set aside.
Preheat the oven to 350.
In a medium mixing bowl, whisk together the eggs, milk, cilantro, and salt.
In a 10- or 12- inch cast iron skillet, heat the olive oil over medium heat. Add the onion and chorizo and sauté until onion starts to become translucent, about 5 minutes. Add the garlic and cook for another minute. Add the spinach and stir until spinach is just wilted. Stir in the barley last.
Pour the egg mixture over the vegetable-barley mixture and stir until combined.
Bake for 25 minutes, until eggs are set and edges are starting to brown. Sprinkle cheese over top and bake for another 5 minutes. Let rest for 5 minutes before slicing into wedges.