In a medium bowl, whisk all the ingredients together in a small bowl and set aside.
Add all of the ingredients, except the cabbage, carrots and cucumber, to the bowl of a food processor fitted with a blade attachment. Pulse until the mixture is blended but its texture is still slightly chunky.
Bring a large pot of water to a boil over medium heat and add the cabbage leaves. Blanch for 30 seconds. Transfer them to an ice bath to stop the cooking and cool. Remove the leaves from the ice bath and arrange them on a sheet tray lined with a kitchen towel.
Put a cabbage leaf onto a clean work surface. Halve the large stem in the middle of the cabbage widthwise so that it is thin enough to be rolled without the stem breaking. Spread 1/4 cup of the seafood mixture over the bottom 1/3 of the cabbage leaf. Tightly roll the leaf up halfway. Fold the sides of the leaf inward toward the center of the roll to keep the filling contained. Tightly roll the sausage up the remaining way. Repeat with remaining ingredients. Make 3 punctures in each roll with a toothpick to allow any excess moisture to escape during the cooking process.
Set up a bamboo or vegetable steamer over a pot of simmering water. If you do not have a steamer, you can use a colander.
Arrange the sausages in the steamer and steam until they are cooked through, about for 10 minutes. Remove them from the steamer to let cool for 5 minutes.
Slice each sausage lengthwise 3/4 of the way through, leaving the bottom still intact. Split the sausages open and top them with the julienned carrots and cucumbers. Drizzle with the mustard and serve.