Put the onion cubes in a saucepan with a little olive oil and raw cane sugar and fry until translucent. Deglaze with the broth. Add sour cream, mustard, the fish sauce and cashew nuts and puree everything finely with a blender.
Preheat the oven to 200 ° C top / bottom heat. Place the skrei in an oiled baking dish, skin side down. Pour the sauce over them and place in the oven for approx. 20 minutes.