For the white wine-steamed mussels: Clean the mussels by rinsing them under fresh running water. Remove the beards (the hairy strands attached to the mussels) and discard any damaged or dead mussels. If any mussels are open, gently tap them on the side of the sink; if they do not close, discard them, as this indicates they are dead. If you clean the mussels in advance, refrigerate them in a colander set above a bowl, covered with a damp paper towel, until ready to use.
Melt the butter in large stockpot over medium-high heat. Add the garlic and shallots, and cook until softened and beginning to color, 3 to 5 minutes. Add the mussels and white wine (the wine should start to bubble immediately). Cover the pot and let the mussels steam for 5 to 7 minutes, stirring once halfway through. You will know the mussels are done cooking when they open; discard any that do not open.
Using a slotted spoon, remove the mussels from the pot and set aside to cool. Discard the remaining liquid. Refrigerate the mussels until you are ready to assemble your platter.
For the white wine and tarragon sauce: Melt the butter in a small pot over medium heat. Add the shallots and garlic and saute, stirring occasionally, until translucent, 3 to 5 minutes. Add the tarragon and cook until wilted, about 1 minute. Add the wine and lemon juice, and season with salt and pepper. Simmer on low heat for about 5 minutes. Taste and add more butter if the sauce is too acidic. Adjust the salt and pepper if needed. Keep warm until ready to serve.
To assemble the seafood platter: Cover a large serving platter with a layer of crushed or cubed ice. Arrange the shrimp, crab claws, langoustines, lobsters and steamed mussels on top. (I like to keep the lobsters and langoustines whole because I think they look better presented like that.) Be sure to crack the crab claws to help guests easily extract the meat.
Garnish with lemon slices. Serve with melted butter and the white wine and tarragon sauce in ramekins on the side.
If you can't buy precooked seafood, you can cook everything in salted boiling water (sometimes I also add a vegetable bouillon cube for more flavor) for the following times: Lobster (1 1/2 pounds): 10 to 15 minutes Langoustines: 5 minutes (do these in batches so you do not overcrowd the pot) Shrimp: 3 minutes Crab claws: 8 minutes (these are also nice steamed)