Cook rice according to package directions and place meat in freezer to firm it up for slicing. When rice is 10-15 minutes away from serving, remove the meat from freezer and cut it into slices no more than 1/8 inch thick.
Combine the tamari and beef stock in a small pot over low heat.
Heat about 2 tablespoons of oil over high heat in a large, nonstick skillet. When the oil starts to smoke, add the meat and stir-fry 4-5 minutes, until caramelized. Remove meat from pan and reserve on a platter.
Heat remaining 2 tablespoons of oil in the same pan the meat was cooked in and add the onion, mushrooms, bok choy, bamboo shoots and scallions, and stir-fry 3-4 minutes.