Mix the mustard, pepper, 1/2 teaspoon salt and 2 tablespoons of canola oil in a shallow bowl. Add the fish fillets, turning to coat. Marinate the fish in the refrigerator for 30 minutes.
Combine the onion, garlic, ginger, and cashews in a blender or food processor and pulse until the mixture forms a paste.
Preheat an oven to 180 degrees C.
Heat 1 tablespoon of canola oil in a frypan over medium-low heat. Stir in the prepared paste; cook and stir for a minute or two. Add the cayenne pepper, turmeric, cumin, coriander, 1 teaspoon salt and sugar. Cook and stir for an additional five minutes. Stir in the chopped tomato and vegetable stock.
Arrange the fish fillets in a baking dish, discarding any extra marinade. Top the fish with the sauce, cover the baking dish and bake in the preheated oven until the fish flakes easily with a fork, about 30 minutes. Garnish with chopped coriander.