To make the vinaigrette, whisk the mustard and lemon juice in a bowl. Gradually whisk in 6 tablespoons of the olive oil. Add the tarragon and garlic clove and leave to infuse. Season, to taste, with salt and freshly ground black pepper.
Pick the meat from the lobsters and set aside in a bowl in the fridge, reserving the shells.
Heat a saucepan over a medium heat. Add the remaining two tablespoons of the oil, chopped fennel, celery and onion and fry until softened. Add the lobster shells and fry for 2-3 minutes.
Add the white wine and bring to a simmer. Add the stock or water, return to a simmer for an hour, then strain the stock through a fine sieve.
Place the liquid back into a saucepan and add the cream. Simmer for 30 minutes, or until the volume of the liquid has reduced by half and thickened. Season with salt and pepper and leave to cool. Chill the lobster cream in the fridge until ready to serve.
Slice the new potatoes into thin rounds. Arrange the rounds in a chefs’ ring onto each serving plate to create a circular base. Remove the chefs’ ring.
Place a Baby Gem leaf onto the potatoes. Lightly dress the lobster meat with a little of the tarragon vinaigrette, then spoon into the Baby Gem. Place a small handful of mixed cress on top of the lobster. Place the diced tomato around the edge of the plate then dress with a little of the cold lobster cream.
The lobster cream and vinaigrette can be made up to a day ahead.