One "lg" can of....
This means, one "large" can of...
"lg" is a (poor) abbreviation for "large", precisely for the reason that it confused you.
I come across these sort of abbreviations in recipes all the time, and they can be exasperating. How large is large? 25 grams or 250 grams?
The best one I ever saw was "cook on a hot fire for 10 minutes". Is there another kind of fire that I don't know about?
Good luck and bon appetit!