Step 1

Adjust oven rack to middle position and heat oven to 350 degrees. Whisk eggs, milk, and 1/2 teaspoon salt in bowl until well combined.

Step 2

Heat oil in 12-inch ovensafe nonstick skillet over medium-high heat until shimmering. Add potatoes, cumin, and 1/4 teaspoon salt; cook, stirring frequently, until half of potatoes are lightly browned, 8 to 10 minutes. Add chorizo, water, and vinegar; continue to cook, stirring constantly, until no water remains in skillet, about 1 minute longer.

Step 3

Add cilantro and egg mixture and cook, using rubber spatula to stir and scrape bottom of skillet until large curds form and spatula leaves trail through eggs but eggs are still very wet, about 30 seconds. Smooth curds into even layer and cook, without stirring, for 30 seconds. Transfer skillet to oven and bake until frittata is slightly puffy and surface bounces back when lightly pressed, 6 to 9 minutes. Using rubber spatula, loosen frittata from skillet and transfer to cutting board. Let stand for 5 minutes before slicing and serving.