The lowest syn I can suggest is a homemade one on a green day using both HEAs. Here's what I do
Boil some elbow macaroni, while its cooking heat up 250ml semi skimmed milk in a saucepan, mix 1 tbsp cornflour (3.5 syns) and stir well until thickened. Season with salt, pepper and a bit of fresh grated nutmeg. Grate 42g reduced fat mature cheese and stir half of it into the white sauce. Add to the macaroni, stir well and pour into a baking dish. Sprinkle with the rest of the cheese and bake for 30 minutes on 180C until golden brown on top.
An even easier method is to make up a packet of batchelor's pasta & sauce (mild cheese and broccoli) and you can then transfer into a baking dish and sprinkle with cheese and bake as above.
If you grate the cheese on a fine microplane grater it goes a loooong way!