In a large wok, heat 2 tablespoon of canola oil over high heat. Season beef with salt and pepper and add to wok. Cook for 2-4 minutes, until meat has browned. Transfer meat to a large bowl and stir in hoisin sauce. Cover and set aside.
Pour out extra liquid from wok. Lower heat to medium high, then add remaining canola oil to wok. Add in garlic, scallions, and ginger and cook until fragrant, about 30 seconds. Add in turnips and bok choy stems. Cook until stems soften, about 5 minutes, then add in bok choy greens. Pour in soy sauce and Sriracha, then cook about 2-4 more minutes, until greens are wilted. Adjust salt to taste.
Serve beef over vegetable mixture. Top with sesame seeds.