Prepare the marinade by mixing soy sauce, brown sugar, sesame oil, garlic, ginger, rice vinegar, Sriracha (if using), black pepper, green onions, sesame seeds, and white grape juice in a bowl.
Slice the steak against the grain into ½-inch pieces and marinate for at least 30 minutes or overnight.
For the spicy cream sauce, whisk together mayonnaise, gochujang, rice vinegar, honey/maple syrup, garlic, and toasted sesame oil in a small bowl until smooth.
Cook rice according to package instructions.
Grill or pan-sear the marinated steak over medium-high heat for about 2-3 minutes on each side until charred and juicy.
Assemble bowls with warm rice as the base and top with grilled steak slices and desired fresh toppings. Drizzle with spicy cream sauce before serving.