Combine flour, sugar, and salt in a bowl and mix in yeast dissolved in 200ml tepid water followed by another 150ml water to form a dough.
Knead the dough on a floured surface for 10 minutes, shape into a sausage, and prove on a tray for at least 6 hours.
Preheat oven to 220°C, dust loaf with rye flour, slash the top, and bake for 30 minutes until hollow-sounding.
Blend softened butter in a food processor until fluffy and transfer to a dish.
Steam langoustines, razor clams, and clams for 8 minutes until cooked, discarding any closed clams.
Halve the lobster, clean the head, crack the claws and knuckles, and return picked meat to the shell.
Open the crab shell, remove gills, crack the claws, and return picked meat to the shell.
Whisk egg yolks, mustard, vinegar, and lemon juice while slowly drizzling in oil to create a thick mayonnaise thickened with zest and seasoning.
Arrange all seafood, oysters, and shrimps on a platter and serve with mayonnaise, butter, and sliced bread.