Step 1

Arrange a rack in the middle of the oven and heat the oven to 425°F.

Step 2

Prepare the following, placing them on a rimmed baking sheet as you complete them: Trim and halve 1 pound Brussels sprouts. Cut 1 pound sweet potatoes (no need to peel) into 1-inch chunks. Cut 1 medium red onion into 1-inch chunks.

Step 3

Drizzle with 3 tablespoons of the olive oil, season with 1 teaspoon kosher salt and 1/2 teaspoon black pepper, and toss to combine. Spread out in an even layer. Roast for 25 minutes. Meanwhile, coat the fish with the pistachios.

Step 4

Place 3/4 cup shelled, roasted, and salted pistachios, 1 garlic clove, and the remaining of 1 teaspoon olive oil in the bowl of a food processor fitted with the blade attachment. Pulse until coarsely chopped, about 40 pulses. Transfer to a wide, shallow bowl or pie plate.

Step 5

Pat 4 skinless cod fillets dry with a paper towel. Lightly season all over with kosher salt and black pepper. Brush 2 tablespoons Dijon mustard on the tops of the fish. Place the fish mustard-side down in the chopped pistachios and press firmly to coat so that they adhere. Place the fish pistachio-side up on a large plate. If there are any pistachios left, use your hands to press them into any open spaces on the fish. Refrigerate until the vegetables are ready.

Step 6

Remove the baking sheet from the oven. Reduce the oven temperature to 400°F. Toss the vegetables, then push them aside to create 4 open spaces for the fish. Use a spatula to carefully transfer the fish to the open spaces. Roast until the fish is opaque and flakes easily, and the vegetables are tender, 10 to 15 minutes more.