Ham-rolled endive gratin

I used to hate this dish when I was little, probably because my mum likes it bitter. But with time, I grew to love it!
Unfortunately it is quite hard to find endives in Britain... So when |I can't find endives (also called chicons), I use leeks.

Take two endives per person, remove the hard core to get rid of the bitterness, cook them for 10 min in the steamer.Roll each endive in a slice of ham. Place in an oven dish.
Cover with white sauce (with salt and pepper), then sprinkle with grated cheese (gruyere's my fav but use what you can).
Place in the oven at 180 degrees for 15 minutes, and it's ready!!

How to make the white sauce:
Place 25g butter and the equivalent in volume of flour. heat up and mix well, cook it for a bit. Then add gradually the milk, still stirring. when all the milk (that is the quantity of white sauce you require) is added, if the sauce is still too liquid, bring to light bol stirring, and then take it off the fire, it's done! Add salt and pepper at the end.

Source: my mum!