Grind the coriander seeds coarsely. Heat them in a saucepan with the oil and allow the aromas to infuse. Peel the potatoes and boil them in salted water. Press them through a sieve and combine with the other ingredients and knead to form a smooth dough. Let the dough rest for 30 mins, then roll it out to 2mm on a floured surface and cut out circles. Panfry quickly in a dry, smoking hot frying pan. Brush the bread with the butter from the lobster and heat them briefly in the oven before serving.
Boil the lobster for 3 minutes in salted water. Remove the claws and boil them for 2 more minutes. Cool the lobster in an ice bath. Shell the lobster; but keep the discarded shell for the mayonnaise. Cut the meat into small pieces. Melt the butter in a saucepan and add the lobster pieces. Heat the butter gently before serving.
Break the lobster shell into smaller pieces and roast for 20 minutes in an oven at 160°C. Crush the shells and heat them in a saucepan with the oil, then strain the oil.
Combine the egg yolk with vinegar and mustard. Add the lobster oil slowly while mixing with a hand blender. Season the mayonnaise with salt, lemon and tabasco. Pour the mayonnaise into a piping bag and keep it in the fridge until serving.
Peel the grapefruit using a knife and cut out the fillets. Slice the celery thinly and chop the spring onion finely.