For the seafood tower, use what is fresh and locally available. The periwinkles are best if made the day before serving.
Poach the periwinkles: Make a sachet with rosemary, thyme, and bay leaf. In a large stockpot over high heat, combine clam juice and sachet and add water until pot is ¾ full. While waiting for mixture to boil, fill your sink or a large mixing bowl with cold water and wash periwinkles to remove mud. Drain and repeat until water is clear. Once poaching liquid boils, add cleaned periwinkles and simmer for 30 minutes, skimming off any foam that floats to the surface. Drain periwinkles and set aside in refrigerator until cooled completely.
Make the mignonette: In a medium bowl, combine all ingredients.
Make the shrimp: In a medium pot over high heat, combine fish stock, lemon, bay leaf, and salt and add water until ¾ full. Once boiling, drop in raw shrimp. After 1½ minutes, remove and drain shrimp. Cool in refrigerator until ready to serve.
Make the cocktail sauce: In a large bowl, combine all ingredients and stir until fully incorporated. Adjust salt to taste.
Make the crab verrine: In a small bowl, combine peppers, crème fraîche, shallot, and lemon juice. Using a spatula, gently fold in crab and mix without breaking up lumps. Season with salt and pepper to taste. Portion into small shot glasses or a medium bowl. Top with caviar.
Assemble seafood tower: Fill a tiered serving stand (or large platter) with crushed ice. Arrange seafood over top and serve with sauces.