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.
While the veggies are cooking, beat egg yolks in a small bowl and pour a little of the warm tamari beef broth into the yolks -- be careful not to add the hot liquid too quickly or you will scramble the egg yolks and have to start over. Once the yolks are whisked up with the hot liquid, add it back to the pot and place over medium low heat, whisking for 2 minutes or until slightly thickened. If you are not comfortable using egg yolks, combine a tablespoon of cornstarch with the warm soy broth then whisk into sauce.
Once the sauce has thickened, add it to the pan with the veggies along with the reserved meat. Toss to combine and heat through.
To serve, top each portion of the stir-fry with a scoopful of brown rice. Garnish each serving with sesame seeds.