In deep half of frittata pan, medium heat, warm 1 Tbsp. olive oil. Add onion and 1/2 tsp. salt. Cook until tender & translucent, about 5 min. Transfer to plate.
In a bowl, whisk eggs, salt & pepper. Stir in potatoes, chorizo and onions. In same pan, on med-low heat, warm 1 Tbsp. oil, add eggs. Cook, loosening edges occasionally with rubber spatula for 5-6 minutes. Adjust heat so underside turns golden, but not brown.
In shallow half of pan, warm 2 tsp. oil over low heat. Place upside down on top of deep pan. Holding firmly, flip and remove deep pan. Put shallow pan over med-low heat and cook, pressing center lightly with spatula until set, 5-6 min. Loosen edges with spatula and slide onto large plate. Let stand 10 minutes. Cut into slices and serve.