Wilted Cabbage with Mustard & Horseradish - Fair Shares

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