Whisk mustard and lemon juice in a bowl before gradually adding 90ml of olive oil, tarragon, garlic, and seasoning.
Extract lobster meat from shells and refrigerate while reserving the shells for the stock.
Fry fennel, celery, and onion in remaining oil until soft, then sauté the reserved shells for 3 minutes.
Simmer the mixture with white wine and 300ml of liquid for one hour before straining through a fine sieve.
Add cream to the strained liquid and simmer for 30 minutes until reduced by half, then chill.
Slice potatoes into rounds and arrange in a circle on plates using a chef's ring.
Top potatoes with lettuce and lobster meat dressed in vinaigrette, garnishing with cress, tomatoes, and chilled lobster cream.