Discard coarse outer leaves from Brussels sprouts.
Then rinse sprouts, drain, and place on a rack above 1 inch of boiling water in a large pan.
Cover pan and steam sprouts over high heat until tender when pierced (about 10 minutes).
When sprouts are almost done, combine sugar, vinegar, mustard, and margarine in a wide nonstick frying pan.
Cook over medium-high heat, stirring, until mixture bubbles vigorously.
Stir in sprouts; season to taste with salt.