Combine soy sauce, mirin, sherry, brown sugar, vinegar, garlic, ginger slices, and peppercorns in a medium saucepan. Bring to a boil over high heat, then reduce to a gentle simmer. Cook until mixture is thick and syrupy, about 25 minutes. Let cool for 10 minutes, then pour through a fine mesh strainer. Set aside or store in an airtight container until ready to use.

In a large mixing bowl add meat, panko, corn starch, scallions, egg, ginger, garlic, sesame oil, salt, and pepper. Using your hands, mix until thoroughly combined. Cover bowl with plastic wrap and refrigerate for at least 15 minutes, up to overnight. Using wet hands, form mixture into 1 1/2-inch long slightly oval meatballs (like tiny meatloaves). Thread 2 to 3 meatballs onto each skewer (if grilling). If broiling, place balls on a lightly greased foil-lined baking sheet).

Prepare a grill or stovetop grill pan for high-heat. When hot, oil grill well and place skewers a few inches apart. Cook until first side is well browned and meat releases easily from the grill, about 3-4 minutes. Using tongs and a thin spatula, gently rotate meatballs and cook until well browned on second side, 3 minutes. Brush sauce generously over meatballs and allow to cook for 15-30 seconds longer, until brown and charred in spots. Transfer skewers to a plate and brush with sauce again until meatballs are well-glazed.

Preheat broiler and set rack 5 inches from heat source. Place the meatballs on a lightly greased foil-lined baking sheet and broil the meatballs, turning and basting every 2 minutes with the glaze, until the meatballs are cooked through and dark golden brown, about 8 to 10 minutes. Transfer to a platter.

Sprinkle meatballs with scallion greens and toasted sesame seeds and serve while hot.