I've found the easiest way is to, in one feeding, feed a mouse first and then offer a rat. I assume she can eat two per meal. Next time try a rat first. Make sure they are similar in size so she's not thrown off.

The most important thing in getting consistent feedings is to have a routine. Mine know when they come out and go directly into a shoe box that its feeding time. If they know its feeding time, the prey shouldn't matter too much.