Preheat oven to 450°F with racks in upper and lower third positions.
Whisk 5½ tablespoons oil, ¼ cup vinegar, 1½ tablespoons herbs, 1½ tablespoons mustard, 1 teaspoon pepper and ½ teaspoon salt in a large bowl. Set aside ⅓ cup of the mixture.
Add sliced squash and onion wedges to the remaining mixture in the bowl; toss to coat. Spread in a single layer on a large rimmed baking sheet. Do not wash the bowl. Roast on the lower third rack until vegetables start to brown on the undersides, about 15 minutes.
Meanwhile, toss halved Brussels sprouts and broccoli with the reserved ⅓ cup oil mixture in the same bowl. Spread in a single layer, cut-sides down, on another large rimmed baking sheet.
Move the squash and onions to the upper rack. Place the broccoli and Brussels sprouts on the lower rack. Continue baking until all the vegetables are tender and browned, 15 to 20 minutes.
Transfer to a platter. Sprinkle with the remaining ⅛ teaspoon salt. Garnish with more vinegar and/or herbs, if desired.
