Wilted Cabbage with Mustard & Horseradish

Ingredients:
3 tablespoons unsalted butter
1 large onion, halved lengthwise and thinly sliced
2 to 2 1/2 lb green cabbage, quartered, cored, and cut crosswise into 1/4-inch-thick strips (12 cups)
3/4 teaspoon salt
1 cup water
2 tablespoons coarse-grain mustard
1 tablespoon bottled horseradish
1 tablespoon all-purpose flour

Preparation:
Heat butter in a 12-inch heavy skillet over moderately-high heat until foam subsides, then sauté onion, stirring occasionally, until lightly browned, about 5 minutes. Stir in cabbage, salt, and 1/2 cup water and cook, covered, stirring occasionally, until cabbage is just tender, about 12 minutes. Transfer to a serving dish. Whisk together mustard, horseradish, and flour in skillet, then add remaining 1/2 cup water and whisk until combined well. Simmer 2 minutes, then stir into cabbage. Season with salt and pepper.