Heat the butter in a medium saucepan, then add the shallot and a pinch of salt. Cook over a low-medium heat for 6-7 minutes until slightly softened but not coloured. Add 65g of the flour, the mustard powder, cayenne and nutmeg, then cook, stirring, for 3-4 minutes until the flour starts to smell biscuity. Turn up the heat and gradually add the stock, stirring all the time, so the mixture thickens and bubbles gently. Once the stock is added continue to add the milk in the same way, then cook for 3-4 minutes, stirring, until the mixture is very thick and comes away from the sides of the pan when stirred. Stir in both kinds of crabmeat and the chopped herbs, then taste and season as needed.

Transfer the sauce to a baking tray or large shallow bowl, smooth the top and cover with a layer of cling film directly touching the surface. Leave to cool completely, then chill in the fridge for at least 3 hours, or up to overnight.

For the romesco, either grill the peppers under the hottest grill setting, turning often until blackened all over, or char them directly in the flame of a gas hob, using metal tongs. Once the peppers are blackened, put in a bowl covered with cling film or in a freezer bag and leave to cool for 20 minutes. Peel off the blackened skins (a quick rinse under cold water will help), remove the stalks and seeds, then tear into strips. Put in the bowl of a food processor with the remaining romesco ingredients (except the olive oil) and season well. Whizz to a thick paste, then pulse in the olive oil until well combined but still with some texture. Taste, season, scoop into a bowl and set aside.

When ready to assemble the croquetas, put the remaining 75g flour and the eggs into separate, wide, shallow bowls and the breadcrumbs into a small roasting tin. Dust your hands with flour, then scoop out walnut-size pieces of the croqueta mixture with a teaspoon and roll into a ball between your palms. Drop into the flour to coat, then into the eggs, shaking gently each time to remove any excess. Finally, put them in the breadcrumb tin. Work until you have 3-4 croquetas in the breadcrumb tin, then shake the tin gently to coat the croquetas all over – squeeze them carefully in your hands to secure the coating. Put the breadcrumb-coated croquetas on a baking tray lined with baking paper, then chill all 24 croquetas in the fridge for 1 hour (or in the freezer for 40 minutes).

Heat the oil in a large heavy-based saucepan over a medium-high heat to 180°C when tested with a digital thermometer. (If you don’t have one, drop a few breadcrumbs into the oil after it’s been heating for about 5 minutes – if they sizzle enthusiastically it’s about right.) Fry the croquetas in batches for 4-5 minutes until deep golden – don’t crowd the pan, and if they stop sizzling at any point, remove them from the pan with a slotted spoon, turn up the heat and bring the oil back to temperature before continuing. Keep the cooked croquetas warm in a low oven while you finish the rest. Serve with the romesco for dipping.