In a mixing bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, and minced ginger. Whisk until well blended.
Place the flank steak in a shallow dish or resealable bag and pour the marinade over it. Allow to marinate for at least 15 minutes or up to 2 hours for more flavor.
While the steak marinates, cook your rice according to the package instructions in a rice cooker or pot.
Prepare the spicy cream sauce by combining mayonnaise, gochujang, rice vinegar, and lime juice in another bowl. Whisk together until smooth and well mixed.
Heat a grill or skillet over medium-high heat. Remove the steak from the marinade and cook for about 4-5 minutes on each side or until it reaches your desired level of doneness. Let it rest for a few minutes before slicing thinly against the grain.
To assemble the rice bowls, start with a base of cooked rice. Top with sliced steak, a drizzle of spicy cream sauce, chopped green onions, and sesame seeds.
Optionally, add vegetables like steamed broccoli or sliced bell peppers for extra color and nutrition.