Wash the potatoes and cut into thin slices. Finely chop the onion. Wash the bell pepper and cut into bite-sized pieces. Heat oil in a small coated pan (approx. 20 cm Ø). Fry the potatoes for about 10 minutes.
Add onions and bell peppers and fry for another 5 minutes. Season with salt and pepper. Mix eggs and milk. Season with salt and pepper. Wash the parsley, pluck off the leaves, chop them and stir into the egg milk. Drain the tuna well and chop it roughly with a fork. Spread the tuna on the potatoes. Spread egg milk over potatoes, peppers and tuna. Cover and let everything simmer at low heat for about 10 minutes.
Drain the tuna well and chop it roughly with a fork. Spread the tuna on the potatoes. Spread egg milk over potatoes, peppers and tuna. Cover and let everything simmer at low heat for about 10 minutes. Cut the omelette in half and serve. Garnish with parsley as desired