Lightly caramelize the onions in the oil.
Add the tomatoes and cook slowly until all the liquid has evaporated.
Add the garlic; turn off the heat and leave to cool in the pan.
Char, peel and deseed the sweet peppers, cut into strips ¼ x 2 inches.
Add the peppers and any juices to the onion and tomato mixture, bring to a simmer and cook gently until well amalgamated.
Season with the vinegar, salt and pepper to taste.
The piperade can be made1 to 3 days in advance and reheated.
Put the shallots, vinegar and white wine in a small non-reactive pan.
Boil to reduce until almost dry, add the cream.
Return to a boil and gradually whisk in the chilled butter.
Allow to return to a boil, remove from the heat continue whisking for a few moments.
Season with lemon juice, salt and pepper.
Keep warm until ready to use.
Stir in the parsley just prior to serving.
Preheat oven to 500 degrees F.
Pat the cod dry with a paper towel and season with salt and pepper.
Heat a large oven proof frying pan, add a little vegetable oil and when very hot place the cod skin side down into the pan and place the pan into the oven.
Roast for about 8 minutes until the flesh is just cooked and the skin nice and crispy.
While the cod is cooking reheat the piperade and add the spinach and olives, allowing the spinach to wilt.
Serve the cod on top of the piperade and pour a little of the sauce around it.