Combine shallots, mustard, apple cider vinegar, and maple syrup. Add the oil slowly while whisking to emulsify the vinaigrette and set aside. Salt and pepper to taste.
BrainSlaw Prep
Cut red cabbage in half, trim root end. Remove a few of the leathery outside leaves to expose leaves that have more folds to mimic a brain. Use a slicer to cut two shallow lines down the middle of the cabbage to create the ridge between the two brain hemispheres. Angle the knife to cut out a "V" in the ridge and remove the cabbage from it. Generously coat the cut side of the cabbage with the dressing.
Add red onion, apples and fennel to the bottom of a Dutch oven and place the cabbage brain on top of them. Baste top of cabbage with dressing generously. Reserve remaining dressing for serving. Add 1½ cups of wine or apple juice to the pot. Cover with lid and roast at 400° for 35-40 minutes. Remove lid and bake, uncovered for 5-10 minutes longer.(Test for doneness with a knife as cooking times vary depending on the size of the cabbage.)
Let cool slightly and then transfer to a serving dish. To serve, cut into wedges and top with roasted onions, apples, fennel and additional dressing if desired.
Notes
Variation of Apple Fennel Onion Slaw from Castellon's Kitchen