Prepare the vinaigrette by adding the olive oil, vinegar, mustard, garlic powder, salt, and some freshly cracked pepper to a bowl or jar. Whisk or shake the jar until the ingredients are combined, then set the vinaigrette aside.

Slice the kielbasa into medallions or half-rounds and add them to a large skillet (12" or larger) or a large, wide-bottomed pot, along with the olive oil. Sauté the sausage over medium heat until the pieces are well browned.

While the sausage is browning, finely dice the onion. Once the sausage has fully browned, add the onions and continue to sauté until the onions are soft and transparent.

While the onions are sautéing, chop the head of cabbage into 2-inch by 1/2-inch wide strips. Add the cabbage to the skillet or pot along with a pinch of salt and pepper. Continue to sauté until the cabbage is tender (check the thickest white pieces for tenderness). To help the cabbage soften, add a few tablespoons of water to create steam within the pot or skillet. Let the water evaporate as you sauté the cabbage.

Once tender, drizzle the mustard vinaigrette over the skillet, starting with just half of the prepared amount. Stir to coat the kielbasa and cabbage in the vinaigrette, taste, and add more if needed. Serve warm.