Preheat the oven to 225 degrees F. Cut the tomatoes in half and pull out the seeds. Line a baking pan with parchment paper. Arrange the tomatoes in a single layer, cut side up, in the pan. In a small bowl, mix together the garlic, basil or oregano, and 2 Tbsp. olive oil. Spoon the mixture over the tomatoes and season with salt and pepper. Bake the tomatoes 2 hours.
Place the tomatoes and liquids from the pan in a food processor or blender. Add the stock and pulse until smooth. Pour the sauce into a small pot and set aside.
Preheat the oven to 425 degrees F.
Line another baking pan with parchment paper. Set in the fish and drizzle with orange juice and 1 Tbsp. olive oil; season with salt and pepper. Bake the fish 15 to 17 minutes, or until just cooked through. While the fish bakes, warm the tomato sauce over medium-low heat. To serve, pour a pool of the sauce on each of 4 plates. Set on the fish, garnish with basil or oregano sprigs, and serve.