In a small bowl, whisk together the soy sauce, water, brown sugar, sesame oil, and crushed red pepper flakes until well combined. Set aside.
Heat a large skillet over medium-high heat. Add the oil and swirl to coat the pan. Add the ground beef and cook for about 5 minutes, breaking it apart with a spoon, until fully browned and no longer pink.
Stir in the chopped onion, garlic, and grated ginger. Cook for about 1 minute, stirring frequently, until fragrant and slightly softened.
Pour the prepared sauce over the beef mixture. Reduce the heat to low, cover, and let simmer for about 10 minutes so the flavors can blend together.
Divide the cooked brown rice evenly among four bowls (about ¾ cup per bowl). Spoon the savory beef mixture over the rice. Top with sliced cucumber, Korean chili paste, sesame seeds, scallions, and crispy fried onions. Serve immediately.
