Put a large, deep, ovenproof frying pan on a medium-low heat and drizzle in the olive oil.
Add the sliced garlic, fry for a minute, then stir in the paprika, chilli powder and tomato puree.
Fry for a minute, then add the chopped tomatoes.
Season generously, turn up the heat a little and cook for about 10 minutes, until the tomatoes have broken down a bit.
Add the fish stock and potatoes, squeeze in the juice of half the lemon, and bring to a boil.
Cover and leave to simmer for another 10 minutes.
Stir in the spinach, cover again and cook for two minutes, until wilted.
Check that the potatoes are tender, then heat the grill to high.
Take the lid off the pan, scatter the prawns on top of the stew, drizzle with olive oil, season and pop under the grill for four to five minutes, until the prawns are pink, cooked through and a little charred.
Meanwhile, finely grate the remaining garlic clove into the mayonnaise, squeeze in the lemon juice from the remaining lemon half and season to taste.
Serve the prawns and potatoes topped with the chopped parsley and a dollop of the aïoli.
