For the short pastry, knead 200 g flour, egg, 1 pinch of sugar, 1/2 teaspoon salt and 100 g butter with the dough hooks of the hand mixer. Then knead with your hands to a smooth dough.
Wrap in foil and chill for about 30 minutes. Clean, wash and cut the leek into rings. Wash and dry fish and cut into cubes. Heat milk, 1 teaspoon salt, bay leaves and pepper in a wide saucepan.
Add the fish cubes and let them simmer at low heat for 2-3 minutes. Carefully lift out and put the milk aside. Put leek into boiling salted water and cook for about 5 minutes. Add the peas 1 minute before the end of the cooking time.
Then pour into a sieve, collect the vegetable water and drain well. Measure 150 ml vegetable water. Peel and finely chop the onion. Heat 30 g butter in a saucepan, fry onion in it.
Wash the potatoes thoroughly, cut or slice them thinly. Place them in a gratin dish like roof tiles. Wash and chop the rosemary and sprinkle over the potatoes with a little sea salt. Sprinkle with 4 tablespoons of oil. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for about 40 minutes.
Grease 4 tartlets (15 cm Ø). Spread the ragout evenly in them. Roll out the short pastry thinly (approx. 37 x 37 cm) on a floured work surface. Cut out 4 circles (18 cm Ø).
Cut a hole (chimney 1.5 cm Ø) in each circle. Place one pastry sheet on each tart tin. Press the edge down. Knead the rest of the dough, roll out thinly and cut out small fish.
Place the fish on the dough plates. Whisk the egg yolk and 1 tbsp. water. Brush the dough plates with it. Bake in the preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 25-30 minutes. Serve with a salad.