Spanish Tortilla de Patatas with Caramelized Onions

1. In a large skillet or pan, heat olive oil over medium and gently fry thinly sliced potatoes until tender and just starting to break apart.
2. Transfer potatoes to a bowl lined with paper towels to absorb excess oil.
3. In a separate pan, caramelize thinly sliced onions until deeply golden and sweet. (You can do this a day before if you want)
4. Remove the paper towels and combine the potatoes with the caramelized onions, 4 eggs, 3 yolks, and a generous pinch of salt. Stir gently and let the mixture rest for 5 minutes so the potatoes absorb the egg.
5. Preheat an 8-inch nonstick skillet over medium heat with a little of the reserved oil. Pour in the mixture, smooth the top, and use a spatula to round the edges.
6. Cook for 1 minute per side for a soft, custardy center, then flip carefully using a plate, lid, or cutting board. Cook longer if you like your tortilla more set.
7. Once golden on both sides, plate and let rest before slicing in. Tortilla should be served at room temperature.