A crunchy slaw unites three winter produce stars: green cabbage, brussels sprouts, and citrus.
Bring 1 1/4 cups water to a boil in a large pot over medium-high heat. Add 2 tablespoons vinegar and the cabbage. Cover, and steam until cabbage has just wilted, about 2 minutes. Using a slotted spoon, transfer cabbage to a colander set over a large bowl. Immediately add brussels sprouts and onion to colander; toss to combine. Let cool 5 minutes.
Whisk orange juice, lemon juice, remaining tablespoon vinegar, dill, mustard, poppy seeds, salt, and pepper in a small bowl. Pour in oil in a slow, steady stream, whisking until emulsified.
Transfer cabbage mixture to a large bowl. Add vinaigrette and apples, and toss gently to coat. Let stand at room temperature 10 minutes, tossing occasionally. Serve.
Slaw can be refrigerated in an airtight container up to 2 days.