Roast the salmon on a baking sheet until just cooked through, about 10 minutes. Let cool, and then break apart with a fork.
Combine the scallops, tarragon, mustard, salt and egg whites in a food processor and pulse until smooth. Mix the scallop mixture with the cooled salmon.
Divide the fish mixture into 6 equal portions, about 1/3 cup each. Unroll a 10- to 12-inch-long piece of plastic wrap and place 1 portion of the mixture in the center. Use the plastic wrap to make a casing by folding the plastic wrap over itself and rolling the mixture into a sausage shape.
Twist the ends until the package is tight, and then tie knots or use small zip ties to secure both ends. Repeat with the remaining mixture.
Bring 4 inches water to a boil in a large pot. Preheat a grill to high heat.
Carefully drop the pescabrats into the boiling water. Let the water return to a boil, and then boil for 10 minutes. Carefully remove the sausages and let cool completely before unwrapping. I like to put them in the freezer for 30 to 40 minutes before grilling, but as long as they are cool, you can grill away. The brats are edible at this point, but the grill will add some scorchy flavors.
Brush the grill grates with oil. Grill the sausages just until they get grill marks and are warm, 5 to 6 minutes.
Serve on the potato buns with malt vinegar, mustard, or mayo. Not ketchup. That's the only rule.