To prepare the beans, first clean them, then cut into the top of each bean and pull towards you to remove the stringy section down the side (if you have really good-quality, thin beans then you won’t need to do this). Cut the beans into 4cm pieces.
Peel the potatoes. Partially cut into them, then pull them apart into chunky, uneven-edged pieces similar in size to the green beans. These potatoes are called ‘cachelos’.
Put 40ml of olive oil into a pan on a medium heat and cook the shallots, then add the tomatoes and cook until paste-like, to make a sofrito. Add the potatoes, bay leaf and sherry or wine, and cook until the alcohol has evaporated. Add the green beans and enough water to cover everything, plus a little extra splash. Season with salt and pepper, then put the lid on and cook until the potatoes and beans are soft – around 20 minutes.
Put the remaining 25ml of olive oil into a separate pan on a low-medium heat, then add the garlic and cook until just golden brown – don’t let it get too brown or it will taste bitter. Turn off the heat, add the vinegar and mix together. Sprinkle the garlic over the stew before serving.