I try to find/feed good single leaf food or mixes without spinach, but have occasionally fed Darwin a spring mix, only half-assed picking out the spinach, without him exploding.
Redfoot food has to be more about the average of a week than every single bite.
Baby lettuces are not very good tortoise food. Spring mix lacks fiber, has a poor calcium to phosphorous ratio, and lacks calcium. Its okay to feed it once in a while as part of a varied diet, but it shouldn't be a staple for any tortoise.
Santa Barbara Mix is vastly superior...as it only contains endive, escarole and radicchio...these are the ONLY greens you should truly rely on...and, of course, collard, mustard and turnip greens! If you can find it...arugula isn't a bad way to go either! Did I mention watercress?