Preheat the oven to 425°F (220°C). Lightly coat a rimmed baking sheet with cooking spray or line with foil.
In a mixing bowl combine the ground beef, panko, 1 1/2 tablespoons bulgogi sauce, chopped ginger, the white parts of the green onions, salt, and pepper. Mix gently until just combined.
Shape the mixture into about twelve 1 1/2-inch meatballs and arrange them on the prepared baking sheet, spacing slightly apart.
Bake the meatballs in the preheated oven for 15 minutes, until cooked through and browned.
Transfer the hot meatballs to a bowl and gently toss with 1/3 cup bulgogi sauce until evenly coated.
Sprinkle the meatballs with sesame seeds and the sliced green parts of the green onions.
In a small bowl whisk together the sour cream, 1–2 teaspoons water, and 2–3 teaspoons Sriracha until smooth; adjust water and Sriracha for desired consistency and heat.
Drizzle the spicy sour cream over the meatballs and serve with rice.