First make the salsa: Heat the oil in medium pan, add the onion and fry for 3 minutes. Add the garlic, fry for 2 minutes then stir in the tomatoes and 300ml cold water. Bring to the boil, add the sugar, season with black pepper and leave to simmer whilst you make the burgers.
Make the burgers. Heat 1 tbsp of the oil in a small frying pan and fry the chopped onion and garlic for 5 minutes until softened. Set aside.
Meanwhile, put the mashed potato in a large microwave-safe bowl and cook on defrost for 3-4 minutes, stirring once, until thoroughly defrosted.
Pulse the drained butter beans in a blender or food processor until roughly minced. Add the beans to the mashed potato with the fried onion and garlic, egg, spinach, cheese and thyme. Mix well, then season to taste.
Divide and shape the mixture into 8 patties, then coat in the breadcrumbs.
Heat the remaining oil in a large non-stick frying pan and fry the burgers, in batches if necessary, for 1-2 minutes on each side until golden. Stir the parsley into the hot salsa and serve with the burgers.
Cook's tip: to extract as much water from the spinach as possible, place in a sieve set over a bowl to defrost. When defrosted, press the spinach firmly with the back of a spoon to squeeze out any remaining liquid.