1. Place the potatoes in a large pan of water and bring to the boil. Cook for 20 minutes until tender. Drain and leave until cool enough to handle, then remove the skins and mash the flesh into a large bowl.
2. Heat the butter in a large sauté pan, then add the garlic, followed by the onion and cook for 3 minutes until softened but not coloured. Add the spinach and cook for 2-3 minutes, until wilted. Leave to cool before tipping into a clean tea towel and squeezing out all of
the excess moisture. Chop it some more if too coarse. Stir the spinach mixture into the mash, along with the thyme and cheese. Season well and leave to cool.
3. For the mayonnaise, place the saffron and garlic in a pestle and mortar and pound with the lemon juice until smooth. Beat into the mayonnaise and spoon into a serving dish.
4. Shape the potato mixture with wet hands into 20 walnut-sized balls, then flour, egg and breadcrumb each one. Heat the groundnut oil in a large deep saucepan over a medium heat to 180ºC and fry the croquettes in batches until golden and crisp, about 4 minutes each batch. Serve the crispy croquettes with the saffron mayonnaise.