Gather the ingredients.

Mix 2 tablespoons oil, mustard, black pepper, and 1/2 teaspoon salt together in a shallow glass bowl.

Add fish and turn to coat. Cover and refrigerate for 30 minutes.

While the fish is marinating, combine onion, garlic, ginger, and cashews in a blender or food processor; pulse until mixture forms a paste.

Preheat the oven to 350 degrees F (175 degrees C).

Heat remaining 1 tablespoon oil in a skillet over medium-low heat. Add cashew paste; cook and stir until fragrant, 1 to 2 minutes.

Add cayenne pepper, remaining 1 teaspoon salt, cumin, coriander, sugar, and turmeric to the cashew paste in the skillet; cook and stir for 5 minutes. Stir in tomato and broth.

Remove fish from marinade and shake off excess; discard remaining marinade. Arrange fish in a baking dish and pour sauce over top.

Bake, covered, in the preheated oven until fish flakes easily with a fork, about 30 minutes. Garnish with cilantro.