Pierce the potatoes in a few places with a fork; microwave until slightly softened, about 4 minutes, then halve lengthwise. When cool enough to handle, grate the cut sides on the large holes of a box grater into a large bowl.
Add the sauerkraut to the grated potatoes along with the herbs, eggs, three-quarters of the scallions, 1 teaspoon salt, and pepper to taste. Stir in 1 tablespoon flour, then form into four 1/2-inch-thick patties. Sprinkle the patties with the remaining 1 tablespoon flour.
Heat the olive oil in a medium skillet over medium heat. Pierce the bratwurst all over with a fork, add to the pan and cook, turning, until golden brown, 5 minutes per side. Meanwhile, heat 1/3 inch olive oil in a large skillet over medium heat. Add the potato patties and cook until golden, 6 minutes per side. Drain on paper towels.
Remove the bratwurst from the skillet and reduce the heat to medium low. Add 1/2 cup water and bring to a simmer, then add the mustard and sour cream and stir until hot. Cut the bratwurst into pieces and serve with the potato cakes, sauce and the remaining scallions.