Wash the potatoes and cook in water for about 25 minutes. Boil the egg hard. Quench and peel. Cook the peas in the broth for about 3 minutes. Add milk, bring to the boil and thicken. Season to taste with salt, pepper and mustard.
Dice the egg and fold in. Cut the potatoes and fill with the egg ragout. Sprinkle with chives. Wash the tomatoes and eat with them.