Preheat oven to 425F/220C.
Slice the chard leaves into relatively thin strips, cutting longer pieces in half, and leek in thin rings.
In a small skillet/frying pan that is safe to go in the oven, melt ½ tbsb of the butter over a medium heat and add the leeks.
Break up the rings and cook for around 5mins until they are starting to brown slightly.
Add the remaining butter then add the flour and mix through well.
Reduce the temperature a little then add the milk about ¼ cup at a time, stirring in well. Once all the milk is added, add the two cheeses and mix through, reserving a little to put on top.
Add the chard a handful or two at a time and allow it to wilt in the sauce, it will darken and go limp after a couple minutes. Keep adding the rest, folding over the wilted chard below to help soften the chard you just added.
Put the last little bit of cheese on top and place the pan in the oven for 15-20min until lightly brown on top. Serve immediately.