Five humble ingredients express rich flavors in this classic Spanish dish. Onions not only caramelize in olive oil, but infuse it, creating a savory base for cooking potatoes and eggs. Slice it pizza-style for a shareable breakfast or tapa.
Preheat oven to 350 degrees F. Using a knife, peel the onion (discard onion skin in a bowl for easy clean-up), then remove both ends and cut in a medium dice. Heat a non-stick, oven-proof skillet over medium heat. Add olive oil followed by chopped onions and 1 teaspoon salt. Stir and cook until the onions are soft and lightly caramelized, about 10 minutes. Stir the onions occasionally as they cook.
Slice the potato (leave peel on), then dice the same size as the onions (cut away and discard any blemishes on the potato). Set aside. Crack eggs into a bowl, whisk and set aside. When onions are lightly caramelized, remove from heat. Place a strainer over a bowl and strain the onions from the oil, reserving oil. Set onions aside. Place oil back into the pan, turn heat to high until oil is slightly smoking, 2-3 minutes (hot oil will keep the potatoes from sticking). Add potatoes and 1 teaspoon salt. Stir and cook until potatoes are soft and golden brown, 7-8 minutes (test for doneness by tasting one!).
Strain potatoes from the oil, reserving oil. Add potatoes to the onions and keep the skillet warm over low heat (It’s important to have no trace of onions or potatoes in the skillet or the omelette will stick). Stir beaten eggs into the potato onion mixture. Turn heat to high, add about 1 tablespoon of the oil back to the pan. When the oil begins to smoke, add egg mixture. Using a rubber spatula gently move the eggs around in the pan to distribute the heat. Turn heat to low and let the omelette set around the edges, 1-2 minutes. Transfer omelette to the oven. It will bake a total of 4-5 minutes. First, to set the eggs, 2- 2 ½ minutes.
When the omelette is set all over, remove from the oven to flip and cook on the other side. Shake slightly to make sure it isn’t sticking to the sides. Using a rubber spatula, gently remove the omelette from the skillet onto a plate. Cover with the other plate and flip over. Using a rubber spatula, gently place back into the pan, bottom-side up and back into the oven, about 2 more minutes.
Remove omelette from oven. Using a rubber spatula, slide onto a plate. Serve.