Choose a fry pan suitable to be placed in the oven, heat the butter until it sizzles. Pour in 3/4 of the egg mixture. With an inverted spatula pull cooked portions of egg from the perimeter of the pan to the centre so uncooked egg can reach the hot surface, tilting the pan and moving it as necessary.
Cook until the bottom is just set.
Sprinkle with the tuna, potato, red capsicum and spring onion and heat through.
Pour over remaining egg mixture. Place under the grill and cook for 2-3 minutes until set.