Thinly slice the potato into large bite-sized pieces. Finely chop the onion, and boil to the required softness. (You can do this in the microwave.) Drain the tuna can.
Line the bowl which you will use as a mould with plastic wrap, add the beaten egg and the ingredients from Step 1, and season generously with salt and pepper.
Cover with plastic wrap, and microwave at 500 W for 3 minutes. When it has cooled, remove the top plastic wrap, and gently lift the layer of wrap lining the bowl to make it easier to remove.
Put a large serving plate over the bowl, then turn it upside down. Remove the bowl and wrap to finish.
Story Behind this Recipe
Spanish omelettes are usually large frying pan size, but I wanted to make a small one easily without any oil, so I came up with this.
Use a bowl that is around the same size as that listed. If the ingredients overflow in Step 2, just remove them. Adjust the cooking time to suit your microwave. This contains simple ingredients, so season it well with salt and pepper You can use satoimo (taro) instead of potato. It's tasty.