Heat the oven to 200°C. Bring the stock, which should not be too salty, to the boil in a medium-sized pan and throw in the chard stalks, bay leaves and garlic. Cook for 15 minutes and drain over a bowl to catch the stock.
Make a béchamel sauce; melt the butter in a small pan, then add the flour, over a low heat. Let it cook for a minute or two without browning, and add the milk graduall