Fillet the cod and cut into four equal pieces. Clean the greens and cut them into cubes. Clean the spring onions and cut them in half.
For the sauce, first reduce the white wine to 1/3. Add the fish stock and reduce again to about 1/3. Mix in the creme fraiche and mustard, bring to a boil. Season to taste with salt and freshly ground pepper. Just before serving, mix in half the butter. Sauté the greens in a little butter, deglaze with a tiny bit of fish stock and add the seasonings.
Put the fish in with the skin side down, season with salt and poach in a closed saucepan at a low temperature for about 5 minutes. Steep the spring onions in the sauce for 3 min, remove and keep warm for a short time.
Remove the peel from the potatoes, make them in salted water and then sauté them in a little bit of butter. Arrange the cod together with the spring onions, the potatoes and the sauce on a warm plate. Serve immediately.
Tip: Cook only with high-quality spices – they are the be-all and end-all of a good dish!