If you're counting calories, then this is a great substitution for full-fat béchamel sauce.
Although technically not a béchamel, as it's not made from a roux base (flour and butter cooked together), it's unlikely most people would be able to tell from tasting it! Also, because it's thickened with cornflour, it's gluten free.
You can also make this LACTOSE FREE by using lactose free whole milk, lactose free mature cheddar and either lactose free spread or butter if you can tolerate it.
This quantity will make enough for a lasagne or moussaka for six people. 306 calories for the full quantity, 51 calories per serving. [Calories in square brackets]. My recipe for Skinny Moussaka is here, for only 339 calories per portion, and packed full of flavour. Skinny Lasagne being written up...!
500ml skimmed milk* 
1 small onion (for flavour, not consumed)
Nutmeg, (freshly) grated 
3.5 level tbsp (15ml) cornflour (USA cornstarch) 
1 level tsp salt (or to taste)
Optional 20g grated Parmesan cheese for flavour, if making for lasagne/moussaka [78 - add on 13 calories per serving]
Optional black pepper 
Reserve 50ml of the milk and set to one side. Peel the onion, and cut off the top and the brown, dried part of the root. Cut a deep cross in the top of the onion, and place in a pan small enough for the rest of the milk to cover the onion (but not come right up to the top of the pan).
Grate in some nutmeg (according to your tastes – I probably grate in a good couple of mm of a nutmeg, but I like to taste it! Go easy at first, you can always add some more later.) and turn the heat on. Bring up to a gentle simmer, stirring occasionally – don’t leave it unattended to heat up because if it boils, the milk will foam right up and spill down the sides of your pan (so if you’re on a gas hob, it will put the flame out). Simmer for 2 minutes, and then turn off the heat and leave the onion to infuse in the milk for ten minutes or so.
If at any point during making the béchamel sauce, you realise the milk has burnt on the bottom of the pan, don’t panic – just don’t scrape the bottom or you'll stir it into the sauce! Simply transfer your sauce by gently pouring it (you can sieve it if you’ve accidentally stirred in a speck or two of burnt material from the bottom of the pan, to catch them) to a fresh pan, and taste it to make sure it’s not gone bitter. 9 out of 10 times, you’ll be able to save it!
Remove the onion with a slotted spoon and discard. Turn the heat back on, mix the cornflour into the reserved cold milk thoroughly, and then add to the pan, stirring (a silicone whisk is good for this - although I'm not a fan of using metal whisks in metal pans), until it comes back up to the boil and the sauce thickens up. Once the sauce has thickened, turn off the heat. Stir in the grated parmesan if using, salt to taste (you might want to just start with 1/2 a tsp to start with, then go from there) and some freshly ground black pepper if desired. Taste and adjust seasoning as required.
For a richer sauce, you can use semi-skimmed or full-fat sauce, add on 13 calories per serving for semi-skimmed milk, and for full-fat milk add on 25 calories per serving. You can add further cheese to taste.