Steam Brussels sprouts until they are fork tender. This is usually 5 to 6 minutes in a stovetop steamer. If you are a microwave steamer, 3 to 3:30 minutes or so.
In a small bowl, combine mustard and balsamic. Stir to combine.
When Brussels sprouts have been steamed, place them in a baking dish in a single layer. You may choose to slice them in half or leave them whole.
Drizzle the mustard balsamic sauce over the top, toss a bit to coat the sprouts.