Mainly crickets, some other whole insects all supplemented with a good quality calcium D3 supplement such as Miner-all Indoor.
If you want to add rare occasional variety which has been proven to be not needed, you could feed a few pinky or fuzzy mice every few weeks.
Do Not feed Dogfood, or catfood, etc or people food, Im telling you this for the good of your monitor. Variety can be good if the variety offered is of useful foods, whole animals are useful foods. This subject on variety comes up by new keepers every week its the same answer, do not confuse "shotgunning"your husbandry as offering a good healthy variety. What is meant by shotgunning is that a new owner is scared that the husbandry issues they face or others have faced were from a lack of wide variety not from the real problems, short cutting husbandry, screen tops, bad substrates, no usable temps, etc.