Truthfully, giving her one medium mouse every 7-9 days IS underfeeding her.

You may not want to hear that, but it is true. At 1 year old, even if she started small, she should have been on small rats for at least 3-4 months. At 3 months old, even if she was tiny, she was still primed for a growth spurt if she'd been fed enough. At that age, even if you had to give her small mice she should have eaten at least every 5 days. If she was severely underweight like you said, a hopper mouse every 3 days would not have been overdoing it for a short time.

Now that she is older, it will be considerably harder for her to catch up, because her growth rate has slowed way down. Do give her the largest size of rat she can comfortably eat every 7 day at least, unless she refuses. She will probably grow some more, but it will take a lot longer.

Gale