Cut the beef into bite-size pieces, and cut the onion into thin slices. Cut the bell peppers into thin strips. Cut the carrot into rectangles or cut diagonally into thin slices. Roughly chop the mushrooms.
Sprinkle the beef with salt and pepper, add grated garlic and rub the meat. Let it absorb the flavour (about 5 minutes).
Heat a pan over medium-high heat, and pour in the oil. Add the beef, and stir-fry until both sides are browned.
Push the beef to one side of the pan. Add the vegetables to the empty space. Stir-fry the vegetables until they become soft and wilted. (Onion, carrot, bell peppers, mushrooms in that order).
When the vegetables are cooked, toss with the meat. Add the yakiniku sauce and Doubanjiang, then toss the meat and vegetables to coat with the sauce. Bring to a boil, and turn off the heat.
Put the rice in a rice bowl, lay Step 5 on top, and sprinkle your favorite toppings, then it's done.