Make the polenta by bringing the chicken stock and milk to the boil. While whisking, pour in the polenta, once everything is mixed together cook over a low heat for 10 minutes before adding the butter and 75g of cheddar cheese. Heat the grill to medium.
While the polenta is cooking heat the olive oil over a medium heat in an oven proof skillet or frying pan. Add the chorizo and cook for a couple of minutes before adding the tomatoes and wait until they've released they're liquid. Add the tomato puree, stir everything together, then tip the mixture out onto a plate.
Pour the polenta into the skillet and spread out into a single layer. Top with the refried beans, smoothing out as much as possible. Add the chorizo mixture to the top of the polenta followed by the 50g of cheddar cheese. Place under the grill for 5-10 minutes, or until the cheese is melted and a little browned.