For the bangers: Using a meat grinder or food mill, grind the shrimp, lobster and garlic together into a large bowl. Then add the parsley, tarragon, seafood seasoning, salt, pepper, nutmeg, egg white and lemon juice to the bowl. Mix well with a spoon, ensuring not to over mix. TIP: Take a small bit of the mix and put it in boiling water for a second, take it out and make sure the seasoning is good. If you need more salt or pepper, this is the time to do it, before you fill the casings. To prepare the sausages, poke a small hole at the end of your casing, and then tie end of casing toward end of casing. Next, fill the casing according to the instructions that come with the equipment you are using, or with a pastry bag. While filling, twist or tie the casing every 5 to 6 inches to form each link until end of the casing, and then tie off the casing. To poach the sausages, in a large saucepan, bring water to a boil, and then reduce the heat to a simmer. Add the links and cook for 5 to 6 minutes. Then remove the sausages, repeat the process with the remaining sausages and then let rest to cool. To sear or crust the sausages, in a saute pan over high heat, add 1 teaspoon of oil, sear the sausages on all four sides, 1 to 2 minutes per side, and then repeat the process until all the sausages have been cooked. While sausages are cooling, begin to prepare your potatoes. In a 1 gallon pot, bring 10 cups water to a boil. Then add the potatoes and cook until fork tender, about 20 minutes. After cooking, remove from the water and strain. Then, using a ricer, process the potatoes through the ricer (or a food mill) into a bowl. If you don't have a ricer or food mill, you can use mixer with a whisk attachment, mixing on high to smooth. After ricing the potatoes, add the butter and season with salt and pepper. Finish with heavy cream, stirring in. For the sauce: In a saucepan over medium-high heat, add the orange juice and onions and bring to a simmer. Reduce the juice to 1/2 cup, and then stir in the horseradish. Remove from the heat and whisk in the butter to finish. Hold warm until serving.
For the sprouts: In a saucepan over medium-high heat, add the oil and warm. Then add the bacon and cook for 2 minutes, stirring throughout. Next, add the onions and again cook for 2 minutes; the onions should be softened. Next, deglaze the pan with wine. Stir and add the Brussels sprouts, cooking until browned on edges, 8 to 9 minutes. Season with salt and pepper. To serve, place potatoes on plates and top with Brussels sprouts, then sausage. Drizzle with the sauce and finish with tarragon.