Have run out of onions would be allright to use leeks, will they fry?
I am sure its will be fine but leeks will fry but the taste is very different, you might want to add some onion gravy granuals to the sauce and add some carrot which will provice more bulk to make up for the leeks which will break down more.
Leeks have a much lower sugar content than onion so you won't get the caramelised flvour or colour but they should work OK.