Sauté the onions and garlic in a tablespoon of oil in a large pan until translucent. Add mustard, paprika, cumin, coriander seeds, turmeric and harissa and toast briefly. Pour in the fish stock and lemon juice and bring to the boil once. Season to taste with salt and pepper.
Rinse the pangasius fillet with cold water, pat dry, season with salt and pepper. Put the fish in the pan and let it simmer in the stock over low heat. Decorate with parsley and serve.