2 tablespoons chopped onion (from 1 medium)
3 tablespoons margarine, divided
2 cups carrots, cut diagonally into 1/2-inch-thick pieces
2 cups Brussels sprouts, halved lengthwise
1/3 cup water
1 tablespoon cider vinegar
Cook onion in 2 tablespoons margarine in a large skillet over medium-high heat, stirring occasionally, until softened, 1 to 2 minutes. Add carrots, Brussels sprouts, 3/4 teaspoon salt, and 1/2 teaspoon pepper and cook, stirring occasionally, until vegetables begin to brown, 3 to 4 minutes.
Add water and cover skillet, then cook over medium-high heat until vegetables are tender, 5 to 8 minutes. Stir in vinegar, remaining tablespoon margarine, and salt and pepper to taste