How to Make It
In a blender, puree the mayonnaise with the preserved lemon, garlic, water, cayenne and 3 tablespoons of the olive oil. Season the aioli with salt and pepper and scrape it into a small bowl.
In a large nonstick skillet, heat the remaining 2 tablespoons of olive oil. Season the cod fillets with salt and pepper and dust with the flour. Cook the fish over moderately high heat, turning once, until golden on the outside and white throughout, about 10 minutes total. Transfer the cod to plates and serve with the lemon aioli.
Preserved lemons are a Moroccan ingredient made by macerating whole lemons in lemon juice and salt until very soft. The lemon pulp is often discarded and the peel is used. They're available at specialty food shops and at kalustyans.com.