How to Make It
In a bowl, combine the shallots and vinegar and let stand for 5 minutes; stir in both mustards. Whisk in 3/4 cup of the oil and season the vinaigrette with salt and pepper.
In a pot of salted boiling water, cook the potatoes until tender, 30 minutes. Drain and let cool slightly, then slip off the skins. Break the potatoes into 1 1/2-inch chunks.
Light a grill or preheat a grill pan. Grill the bratwursts over moderate heat, turning, until heated through, 10 to 15 minutes.
In a large nonstick skillet, heat the remaining 2 tablespoons of oil. Add the potatoes and thyme and season with salt and pepper. Cook over moderately high heat, turning, until golden and crisp, 6 to 7 minutes. Remove the pan from the heat and add 1/2 cup of the vinaigrette and the parsley. Season with salt and pepper and toss to evenly coat.
Arrange half of the Citrus-Spiced Red Cabbage on a serving platter. Scatter the potatoes and three-fourths of the watercress over the cabbage and top with half of the bratwursts. Repeat with the remaining cabbage, watercress and bratwursts. Pass the remaining vinaigrette at the table.