You do need a pan that you can put in the oven to make this as I did but, if not, you can use a small quiche dish or similar.
Ingredients to make two portions (I expected it to make one but it was plenty for two)
about 1 tsp veg oil
one small onion, peeled and thinly sliced
a third of a red pepper, diced
two mushrooms, peeled if necessary and sliced
some cooked potato, sliced
two large eggs
a splash of milk
a squidge of garlic puree
30g strong cheddar, finely grated
1/4 tsp mustard powder
chives, finely chopped
Preheat oven to 180C fan.
In the oven proof pan but on the hob, add the oil and soften the onion. Add the red pepper and soften that, then add the mushroom and potato and gently heat, spreading it evenly across the pan.
In a bowl, add the egg, milk, garlic, pepper, mustard and chives and whisk well. Then add 20g of the grated cheese and mix it in.
Pour the egg mixture over the veg in the pan and give it a little shake to spread evenly. Sprinkle over the remaining cheese.
Pop it in the oven to cook through and brown. It took me about 15 to 20 mins.
Slice into quarters and serve with a salad.
Apparently, the difference between a tortilla and a frittata is that the tortilla is cooked completely on the hob and is turned half way through while a frittata is finished off in the oven. That's what Google said, anyway!