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Take a read here on grocery store foods. Once Winter sets in, you will have to grow your own and use grocery greens..
Russian tortoises need broadleaf weeds. You will need to learn about the weeds near you and which ones are okay to feed or not. Pics can be posted here on the forum in the "Plant ID" section, or you can take samples to a local nursery. Look for dandelion, sow thistle, mallow, filaree, hawksbit, and many more. If you are not 100% sure that your source is free of pesticides and other chemicals, don't feed anything from there.
If you must use grocery store foods I like to favor spring mix, endive and escarole heavily. Also add in carrot tops, cilantro, mustard, turnip and collard greens, and lots of other leafy greens.
It is a good idea to add a product called "Salad Style" to grocery store greens to add some fiber: http://www.tortoisesupply.com/salad-style-food-topper/
This is a new food topper and I like it a lot so far: http://www.tortoisesupply.com/HerbalHay
You can also feed them grape, mulberry and hibiscus leaves, tender young spineless opuntia cactus pads, Mazuri tortoise chow and ZooMed Grassland tortoise food. Alternating and mixing up any of these foods will give your russian a good diet and meet its nutritional needs.
I don’t know if they pull out plants specifically by season but the Tortoise Table is UK based and has examples of common yard plants to feed at the top of their site. There are some potential house plants as well. Most people look at this site to use their database on plant safety but the “front” portion is worth a look.
You could also take a look at Shelled Warriors, which is UK based. I don’t know that any of their seed mixes will work in winter but they will have options for you for pellet supplements (no shame in feeding pellet food in the winter; it’s got fiber and other things your tortoise can’t get from salad mix). They also have dried flowers and herbs which can help bulk up the grocery store plants you may need to rely on. Right now I see they are selling Pre Alpin Testudo as their pellet brand, it’s a good one. For dried items they sell a bunch of things but it would probably be easiest to start with the smallest bag of their mixed dried herbs (it’s actually all weeds) and another of their mixed dried flowers.