Put the potatoes in a large pot and cover with cold water. Bring to a boil, over medium heat, and heavily salt the water. Reduce the heat and simmer until tender, about 20 minutes. Steam the cabbage in a steamer sitting over the potato pot. They should be ready at the same time.
Meanwhile, brown the sausage on all sides in a large saute pan over medium-high heat. Remove the sausage to a plate once it has browned. Add the onions to the pan season with salt and pepper, to taste. Cook the onions until caramelized, adding water, a teaspoon at a time, to deglaze the pan. Stir in the Dijon mustard and 3/4 cup water and return the sausage to the pan to finish cooking. Simmer until the sausage is cooked through and water has reduced to a glaze over the onions, about 7 minutes.
Drain the potatoes, reserving some of the cooking water. Rice the potatoes into a bowl and add a bit of the potato cooking water to soften. Season with salt and pepper, to taste. Fold in the steamed cabbage and taste for seasonings, adjusting as necessary. Transfer the bangers and mash to a serving dish and dig in!!!!