In a large skillet, cook bacon over medium-low heat until crisp. Remove to paper towels to drain; reserve drippings.
Meanwhile in a large bowl, combine teriyaki, hoisin, onions, garlic, cumin and pepper. Add beef, mixing lightly but thoroughly. Shape into eight 1/2-in.-thick patties. Grill burgers, covered, over medium heat until a thermometer reads 160°, 5-7 minutes on each side. Keep warm.
In the same skillet, cook mushrooms in 2 tablespoon bacon drippings over medium-high heat until golden, 5-7 minutes. Add bok choy to skillet; cook until tender. Remove and keep warm.
In the same skillet, fry eggs in remaining bacon drippings over medium heat until desired doneness. Serve burgers on buns with bacon, bok choy, mushrooms and fried egg.
Test Kitchen tips
Bok choy is a tender, flavorful green that needs just a short amount of cooking to get it tender, especially the baby variety. Feel free to substitute any favorite greens if you can't find them at your market.
Instead of putting everything on a bun, try adding some of it to a bowl of prepared ramen soup.