Heat the oil in a large frying pan and fry onions on a medium heat for 3 minutes until they start to turn translucent. Add the chorizo, turn up the heat and cook for 2-3 mins until it starts to release it's oil and brown slightly.
Add in the tinned tomatoes, stock, tomato puree, sugar, thyme, salt and pepper and simmer for 7-8 mins.
Add in the butterbeans and cook for 2 minutes until warmed through.
Ladle into two bowls and top with crumbed feta and watercress.