You can substitute onion and/or garlic for the shallots if you want to. If using fine salt, use 1 1/4 teaspoons instead of 2.
You can substitute any white fish for the sole. Makes sure your fish is well chilled; cold meat makes better sausage.
This recipe makes a smooth-style sausage. If you want something with a coarser texture, simply make the sausage mixture as shown, but then fold in a few handfuls of chopped shrimp, scallop, or any other seafood. Once it's cooked and sliced, you'll see pieces of whatever you added studding the link.