In a saucepan, mix together the wine, water, vinegar and shallot. Heat over medium heat until the liquid has reduced to a bit more than half.
On a cooking sheet, place the cod filets.
Coat with olive oil and season with salt and pepper.
When the liquid reduction is completed, cook the fish in the oven for about 15 minutes.
Meanwhile, with a whisk, gradually add the butter to the reduced liquid, whisking constantly to emulsify. (Maintain a slight boil during the process - if the heat is too high, it won't emulsify.)
Pass the sauce through a sieve.
With a spoon, squeeze the shallot to extract as much flavour as possible.
Add the tarragon and Cayenne pepper.
Season with salt lightly.
Keep warm.
Remove the fish from the oven, place the filets on serving plates and top with a generous amount of sauce.
Serve immediately with mashed sweet potatoes and wilted spinach.