I feed mine chicken which is baked on its own in the oven. Nothing added. When its cold I'll give Alan about a level teaspoon every 6 weeks. Other RF owners probably will advise you more of what they do soon
I give my Cherry Head's boiled chicken every three weeks, but I'm always seeing them eating a slug or a worm in their garden. I've also seem some eating a dead baby bird that fell from a nest.
Boiled or baked chicken, same with some fish, worms red/earth, snails, pinkie mice, croc chow, and even hard boiled eggs they go nuts for. I give croc chow its the easiest and favored heavily. Every 3-6 weeks. Just depends on food regime. Wild they would take the opportunity to eat carrion but not readily hunt for it. So I try to mimic a wild like food regime. Offer varieties here and there with certain species.
What to offer? They will eat old carrion. There is not much we can offer, meatwise, that is actually bad for them unless it is very fatty, salty, or heavily processed. Pretty much any bugs/insects/worms/slugs (a favorite of most!); poultry- fresh, ground, cooked, whatever; red meats- just not too much because of fat; fish, especially 'oily fish', even things like goldfish; feeder rats, mice, or chicks, live, fresh killed, or frozen/thawed- great!; eggs, cooked/boiled enough to not be messy; dog or cat foods- the fatty and processed warnings apply, but in small amounts it is not a problem.
How often? This is tricky. There are two things going on here.
1. In the wild habitats, the plants are mostly low in nutrients (rain forests LOOK like they have lots of nutrients, but most of the soil nutrients are washed away), so they eat a ton of fruit and any 'meats' they can- oddly enough, the most common wild meats are butterflies and termites. They are much better evolved to handle fats and proteins than grassland tortoises.
2. Captive diets offer plants much richer in protein than the wild diet offers. This is a problem for grassland tortoises, but helps forest species.
They probably don't need a lot of actual meat considering the proteins they get from grocery store or backyard plants. Offering them a fairly small amount every week or two is probably safe and appropriate. It would also probably be fine to let them gorge once in a while then no more meat for a few weeks if you want.
My herd got a 'meat treat' every two weeks for most of that day's meal. Frozen/thawed pinkies, Superworms, worms, slugs, chicken bits, tuna, mackerel, and egg were my usual preferences.