Put the potatoes and chickpeas into a large pan with about 750ml of the stock, and simmer, covered, for 10 minutes. Put the fresh or frozen spinach leaves on top and cover the pan. Fresh leaves will soften quite quickly and collapse in the steam. Stir them in. Add vinegar and salt, and cook for 10 minutes more. Heat the oil in a frying pan and fry the whole garlic cloves and the bread over a medium-high heat, turning them over until golden brown. Keep a careful watch, as they brown very quickly. Drain on paper towels then put in the food processor with the pimentón, cumin and ground chilli pepper, if using, and blend to a fine paste. Add the hard-boiled egg yolks and blend again. Gradually pour in the remaining 250ml stock and blend to a thin cream. Pour this into the soup, stir well and check the seasoning. Cook for another 10 minutes, then stir in the chopped hard-boiled egg whites and heat through. Before serving, add some water if necessary to thin the soup a little – although it is meant to be thick.