Place the flank steak in the freezer for 20 minutes to firm up, then slice it very thinly against the grain.
In a small bowl, whisk together the coconut aminos, pear puree, minced garlic, grated ginger, black pepper, and toasted sesame oil.
Toss the sliced beef with the marinade and let it sit for at least 15 minutes.
Cook the brown rice according to package instructions; when finished, fluff and stir in the cauliflower rice to steam through.
Heat a large cast-iron skillet over high heat. Once smoking, add the beef in a single layer and sear until caramelized and cooked through, about 2-3 minutes.
Remove the beef and quickly sauté the bok choy in the same pan with a splash of water until bright green and tender-crisp.
Serve the beef and bok choy over the rice mixture, garnished with sliced green onions and sesame seeds.