Heat up olive oil in a 10-inch non-stick pan over medium heat until it begins to simmer. Turn your heat down to low and add in the radish and onion slices, seasoning with a bit of salt as you layer them on the pan. Stir occasionally until tender, silky smooth and fully cooked (37-45 minutes, depending on size and thickness). Do not let the radishes or onion brown.
Lightly whisk your eggs with a generous pinch of salt in a medium bowl, while the radishes are cooking. Set aside.
Drain fully the radishes and onions once cooked, reserving 2-3 tablespoons of the oil. Allow to cool for a couple of minutes (so as to not scramble your eggs). Pour in your eggs.
If using a skillet
Preheat oven to 350°F/180°C.
Wipe out skillet and grease with reserved oil. Pour in egg and radish mixture and cook until the sides begin to set (3-5 minutes).
Remove from heat, sprinkle with toppings (if using any), and bake for 10-12 minutes, or until fully set. Allow the tortilla to rest for 10 minutes before serving.
If using a non-stick pan
Lightly grease a non-stick pan with reserved oil. Pour in egg and radish mixture and cook, swirling and shaking the pan rapidly until the sides begin to set (3 minutes). Turn off your heat and cover your pan for 5 minutes (your tortilla will continue to cook, without burning).
Take pan off the heat and over the sink: place a large plate on top of the skillet, set a hand on top, and invert the tortilla onto it.
Add one or two more tablespoons of olive oil to the pan, and slowly slide the tortilla from the plate back onto the skillet. Cook for 3-4 more minutes, until the second side is lightly browned but the tortilla is still tender.
Invert the tortilla back onto a clean plate. Allow it to rest for 10 minutes before serving.
Please note that one serving size is 1/8th of the keto tortilla española.