I had this super amazing caramelized onion tortilla in Madrid at Juana la Loca a couple years ago and had to recreate it. My first attempt was a fail. It tasted good, but I had never caramelized onions before and did not realize how much they would shrink. The original recipe is all caramelized onion bound by a bit of egg. I only used 2 onions. Ok, so it was more of a tortilla with caramelized onions instead of a caramelized onion tortilla. It was still good and it was my first time at the caramelized onion rodeo, and luckily I had done a test run fail prior to making it right for my big recreation of Madrid tapas at a subsequent dinner party I had planned. To get this right, you need at least 1 1-1/2 onion per egg. At least.
Fast forward 2 years and caramelized onions are a sorta pantry staple. You can make a whole mess of them when you have a couple hours and freeze them. Use them in this, make a fast soup, pizza, a tart – wherever – they are that good.
This tortilla is not a recreate, it is a riff. I decided to add a bit more veg – spinach and potato – and some cheese. I also did not flip it nor did I mix it all together like a traditional tortilla, for the Juana la Loca version, I did mix all together. Anyhoo. Feel free to leave the cheese out if you want it dairy free.