I need to fatten my kitty up..

[ample]

P's cat M'Lady has been very sick. She lost half her weight because she was having trouble eating. I want to fatten her up, her appetite is back with a vengeance. Basically she has a sore in her mouth so she's on antibiotics for a few more days. If it clears up, she's all good but they aren't sure if there's some underlying condition that caused the sore. The vet says it might be thyroid related, which is common is older cats. She's 13.

Any suggestions on foods I can give her that'll put some weight back on her but won't aggrivate her mouth sore?