How to Make It
Preheat the oven to 275°. Put the halved tomatoes on a rimmed baking sheet and toss with 2 tablespoons of the olive oil, 1 tablespoon of the chopped parsley and 2 teaspoons of the vinegar; season with salt and pepper. Turn the tomatoes cut side up and bake for 1 hour. Turn the tomatoes over and bake them for 1 hour longer.
Increase the oven temperature to 300°. Peel the warm tomatoes and reserve the skins. Cut the tomato halves in half and transfer to a plate, along with any accumulated juices. Put the tomato skins on a pie plate and bake for about 15 minutes, turning once, until crisp.
In a bowl, combine 2 tablespoons of the olive oil with the remaining 1/4 cup of chopped parsley and 1 teaspoon of vinegar. Season the vinaigrette with salt and pepper.
In a small saucepan of boiling water, cook the spinach for 30 seconds. Drain and lightly squeeze out the excess water.
Put the sumac in a shallow bowl. Season the fillets with salt and pepper and dredge them in the sumac to coat completely. In a large nonstick skillet, heat 2 tablespoons of the olive oil until shimmering. Add the fillets and cook over moderately high heat until browned and crisp and barely cooked through, about 4 minutes per side. If the fish is browning too quickly, reduce the heat to moderate toward the end of cooking.
Meanwhile, in a medium saucepan, heat the remaining 1/2 tablespoon of olive oil. Add the spinach and the tomato quarters and any accumulated juices and cook over moderately high heat, stirring, until hot. Season with salt and pepper; keep warm.
Slice the remaining 4 whole tomatoes into wedges and arrange them on 4 plates. Drizzle the tomatoes with the parsley vinaigrette. Arrange the spinach and quartered tomatoes on the plates and top with the black cod fillets. Break the tomato skins into large pieces and scatter over the fish. Garnish with the parsley leaves and serve.