Look into doing a banana island. It's were you just eat a single kind of high-calorie fruit, like bananas, for a given amount of days to give you enough calories to fuel your body while being the most gentle on your digestion so it can heal.
I did one for about 5 days, averaging 35 bananas a day, and was able to start eating more food afterwards leading me to gain back some weight I lost when I first started eating my accidental low-calorie plant-based diet from not knowing how to eat plant-based properly.