Most tapas bars in Spain serve Riñones al Jerez, though at home it can be served with rice or pasta as a main meal. You can add sliced mushrooms to increase the number of portions.
Fry the onion in 2 tablespoons of oil over a low heat in a big frying pan. When it starts to soften add the diced bacon and garlic.
Remove the membranes and cut out the middle cores from the kidneys, then cut them into large dices.
Remove and reserve the onion and bacon from the pan and add 1-2 tablespoons more olive oil.
Put in the diced kidneys, a handful at a time, over the highest heat and stir occasionally. When they are sealed, pull them to the sides of the pan and add the next handful. When they are all sealed and coloured, return the onions and bacon, sprinkle with flour and stir in.
Add the Sherry, tomato concentrate and thyme and bring to a simmer. Season to taste.
Tapas are Spain's greatest food invention. "Eat when you drink, drink when you eat" is the philosophy. Spanish men traditionally drink outside the home and rarely alone. They are not meant to be a meal (although a ración is a substantial portion). One tapa per person and a different one with each drink is the idea, then everyone enjoys tasting and sharing.