Sturdy mackerel can hold its own against strong flavors. Here, it's teamed with a sweet-and-savory combination of dates, cherry tomatoes, cilantro, lemon, and spices.
Preheat oven to 400 degrees. Stir to combine butter and 2 tablespoons date syrup in a small bowl. Brush the exterior of each fish with butter mixture until well coated. Season the cavities and exteriors of each fish with salt and pepper.
Heat a large grill pan over medium-high. Cook fish, turning once, until lightly charred, 2 to 3 minutes per side. Transfer fish to a parchment-lined baking sheet and roast until fish is cooked through, 12 to 15 minutes.
Place tomatoes, chopped cilantro, dates, oil, lemon zest and juice, cumin, paprika, red-pepper flakes, and remaining 2 teaspoons date syrup in a medium bowl. Season with salt and pepper. Mix until thoroughly combined.
To serve, transfer fish to a large platter and top with tomato-cilantro mixture. Garnish with cilantro sprigs and lemon rounds.