I diced about 2 TBSP of yellow onions. In a small skillet, I sauteed them with 1/2 tbsp of olive oil. I cut off 1/4 of a kielbasa sausage and cut it in 1 inch pieces and tossed in to heat. Meanwhile, I finely chopped up 8 brussels sprouts. I added the brussels sprouts and while they were cooking, I made a mustard sauce with 1 TBSP balsamic vinegar, 1 TBSP of mustard and 1 TBSP of olive oil. I whisked them together and poured on top of the brussels sprouts and then continued to saute/heat until done. Add salt and pepper. About 15 minutes total prep and cooking time.
This is best of the brussels sprouts are only cooked until they begin to soften, but not until they become tender. You want them to keep the bright green color and to retain some toothiness. It may be more mustardy than some people like, but you can vary the mustard sauce to your own preferences.