Mix the flour, salt and fennel seeds in a bowl. Dissolve the yeast in the water. Mix the dry ingredients with the water and yeast and work until you have a nice dough. Cover the bowl and prove for 40 minutes. Roll out thinly on a silpat mat. Punch out dough circles with a pastry cutter according to instruction and bake for about 15 minutes in 200°C
Peel and boil the potatoes. Allow to cool a little. Mix the potatoes with the egg yolk and pour the oil slowly while mixing. Season with salt and lemon juice.
Pour all the dairy products into a saucepan and heat gently, season with lemon zest, lemon juice, salt and Espelette pepper. Cover with plastic film and set aside
Boil the mustard seeds in salted water for 3 minutes, rinse with cold water. Slice the cucumber thinly on a mandolin and season with salt. Before plating; mix the cucumber with the mustard seeds and the pickling liquid.
Boil the potatoes in salted water just before serving the dish. Cut the matjes herring as instructed. Pour the trout roe and the Crème fraiche into separate piping bags and place in the fridge; chop the chives thinly. Pick the dill and chive flowers into ice water.