Russians do well on a diet of coarse greens and forbes, clovers. Many can be gathered in fields etc. Many of our "weeds" are from Eurasia and may be native foods. A couple good sources are the San Diego Turtle & Tortoise Society and the California Turtle & Tortoise Club web-sites, they have food lists that I have had good success with and agree with. Dandelions are a wonderful food if available. I feed some supermarket greens in the late Summer when my garden is exhausted and drys up. Turnip greens and a few collards. Romaine lettuce as a treat.
Avoid feeding fruit, except maybe cactus pears or small quantities as a treat. High fiber, low to moderate protein (7%?), High Calcium, low Phosphorus seems to be the way to go. Mine eat a lot of prickly pear which is easy to grow in the Western States and elsewhere. Low protein, I use it to "buffer" higher protein source foods. Nutrition of food is an important topic, and they will certainly eat things not good for them. Many captive tortoises are starving and languishing on inadequate diets of lettuce, bananas etc., so good for you to learn more. I grow Allysum, a small and hardy flower which they love and is probably a native food. Often the first thing eaten by hatchlings here. Check out the web-sites and post Q's. Good luck,Vic H.