Cut the cabbage into quarters then cut out and discard the core from each quarter. Halve each quarter lengthwise for a total of 8 wedges. Separate the leaves, keeping the wedges intact, and place on an 18-by-13-inch rimmed baking sheet. Halve the fennel lengthwise, cut out and discard the core, then slice the fennel crosswise into half-moons 1/4 inch thick. Reserve the fennel fronds for garnish. Add the fennel and eggplant to the baking sheet.
Combine the scallions, olive oil, vinegar, Dijon, 2 teaspoons salt and 1 teaspoon pepper in a small bowl. Stir together then pour over the vegetables. Toss to coat and spread into an even layer.
Liberally season the chicken with salt and pepper then place on top of the vegetables, spacing the thighs 2 to 3 inches apart.
Roast in the oven for 30 minutes. Rotate the baking sheet and continue to roast until the vegetables are softened and charred around the edges and the chicken is cooked through with golden and crispy skin, about 30 minutes more.
Remove from the oven and sprinkle with the parsley, oregano and reserved fennel fronds.
Look for a Dijon mustard that has no sugar or white wine in the ingredients.