Place a 9-inch (23 cm) cast iron pan on the middle rack of the oven. Preheat the oven to 400°F (200°C). Keep the pan in the oven until ready to serve.
In a large non-stick skillet over medium-high heat, cook the spinach in the vegetable oil until wilted. Add the sesame seeds, then season with salt and pepper. Set aside on a plate and keep warm.
In the same skillet, soften the zucchini in the sesame oil. Season with salt and pepper. Set aside on a plate and keep warm.
In the same skillet over high heat, brown the meat in the sesame oil, breaking it up with a wooden spoon as it cooks. Add the white parts of the green onion, garlic and ginger. Cook for 1 minute. Add the brown sugar and soy sauce. Cook for 3 minutes or until the meat is cooked through. Remove from the heat.
In another non-stick skillet over low heat, fry the eggs in the vegetable oil.
Remove the cast iron pan from the oven. It will be extremely hot. Brush with the sesame oil, then place the rice in an even layer over the bottom of the pan. On top of the rice, arrange the spinach, zucchini and meat. Place the eggs overtop. Sprinkle with the remaining green onions and serve with the pickled bean sprouts, pickled mushrooms and gochujang sauce.