One head/two heads of lettuce, cabbage, brocolli, cauliflower. You can have two cabbages, but not two lettuces/brocollis/cauliflowers. (Well, maybe you can have two cauliflowers - I'm not sure.)
One squash, two squash (I think "es" is optional)
one onion/potato/bell pepper; two onions/potatos/bell peppers