In a medium saucepan over medium-high heat, add butter sauté the onions and garlic lightly until soft. Add the peas and chopped tarragon, fill with 1/2 cup water and bring to a boil.

Season with salt and pepper to taste and cook until peas are soft but still green. Add cream, adjust seasoning with salt and pepper and add lemon to taste.

Using a hand-blender, mix until smooth. 

Divide the thick, bright green purée between 8 glass verrines (small thick-glass containers) and chill in the refrigerator for 2-3 hours, or overnight. 

Cook the crab in simmering water for 10 minutes and ice down to chill.

Poach the shrimp in court-bouillon until tender (about 90 seconds).

In a separate pot over medium heat, add butter, white wine, shallots and garlic, and simmer clams and mussels until the shells open up. Chill down and reserve the cooking juices. Separate the meat from the shells.

Clean the crab meat out of the shell and make a stock with the crab carcass. Strain the stock and mix with the clam and mussel broth, heat and add the bloomed gelatin sheets to 400ml of stock to form gelee. Strain and chill. 

In a separate pot over high heat, blanch carrots, celery and fennel in salted water till soft. Add the parsley chiffonade and chive tips, mix together with the shellfish meat, vegetables, double-shucked peas and the warm gelée. Divide the gelée over top of the pea purée inside your the glass verrines. Refrigerate.

In a medium saucepan over medium-high heat, add butter and spring onion and sweat until soft. Add cooked potato and cream, reduce by half. Purée quickly in the blender until smooth. Allow to cool and fold in whipped cream. Add salt and lemon juice to taste.