Gut-loading crickets
I feed my crickets with a staple of rolled oats but I round it out with other foods that I feed on occasion.
- For protein and fatty acids I use peanut butter, cat food, and sometimes pieces of leftover meat like chicken or ground beef.
- For sugar, vitamin C & D and moisture I use applesauce, grapes, apples, peach, and banana.
- For additional vitamins, minerals like Nitrogen and Iron, additiional protein and moisture, I use cabbage, carrot, celery, and sometimes spinach or Swiss chard.
The main source of calories will probably come from the rolled oats, though as mbetournay said, you can also use potato to that effect.
I buy all of my crickets and I buy two dozen at a time so additional foods are more of a one-time thing for each batch of crickets I get. And I go through them in two to three days so the crickets are gone before the food rots. But I can always rely on rolled oats because it never rots unless it is wet and, if I buy a big bag of it, I will always have more in case what I put in with the crickets gets eaten.
If you are breeding crickets, you will want to replace their additional food much more often because they will be around longer and the food will rot. In my oppinion you can use pretty much any food you have lying around as long as it has one of the four requirements: protein, sugar, vitamins & minerals. Fruits and vegetables can supply vitamins, and leafy greens, nuts and meat can supply protein. You need fruit for sugar which adds additional calories for daily activity and you need vegetables for minerals like Nitrogen, Phosphate and Iron.
In general, any balanced diet will do. But it needs to be balanced because, not only are your crickets benefitting from the added nutrition, your lizards are too. Empty calories aren't going to cut it nutrition-wise so don't forget to add something extra.