Center the oven rack; preheat oven to 425°F. For easy clean up, line a large baking tray with aluminum foil.
Arrange the squash and Brussels sprouts in a single layer on the baking sheet. Drizzle the veggies with olive oil and sprinkle them with salt and pepper. Toss to coat evenly. Roast the veggies in the preheated for 25 minutes.
While the veggies are roasting, marinate the fish: In a medium bowl, stir together the mustard, maple syrup, and a pinch each of salt and pepper. Add the fish, coating both sides with the marinade mixture. Refrigerate until the veggies have finished their 25 minutes of cooking time.
Move the partially-cooked vegetables to opposite sides of the baking sheet, making an opening in the center of the sheet. Arrange the fillets in open space, and bake until the fish is done but not overdone -- 10 minutes. Plate and serve.