Line a sheet pan with parchment.
For meatballs, combine pork, onions, garlic, ginger, coriander, and soy sauce in a bowl; mix until just combined. Pinch off about 1 tablespoon of the mixture and roll into meatballs, placing each on the parchment paper lined sheet pan. Place tray of meatballs in the refrigerator while preparing the glaze.
Preheat the oven to 400 degrees F (200 degrees C).
For glaze, add sesame seed oil, soy sauce, honey, hoisin sauce, rice wine vinegar, and chili flakes to a small saucepan over medium heat. Bring to a simmer, then lower the heat. Stir frequently as glaze thickens. Set aside about 1/4 cup for dipping. Keep warm.
Bake meatballs in the preheated oven for about 12 minutes. Remove from the oven and turn meatballs over; return to the oven for 3 minutes. Remove from the oven. Gently pour sauce from the saucepan over meatballs, turning to coat thoroughly in sauce, or use a pastry brush to ensure meatballs are fully coated. Return to the oven; bake until glaze is fully on the meatballs, about 5 minutes more. Remove from the oven and keep warm.
For noodles, bring a pot of lightly salted water to a boil and cook udon according to package directions. Rinse under cold water, then set aside. Stir soy sauce, rice wine vinegar, sugar, and sesame seeds together in a small bowl. Set aside.
Set a skillet over medium-low heat. Add chili paste and stir until fragrant, about 30 seconds. Add green onion and stir just to combine. Add noodles and mix to coat noodles with chili paste. Pour in noodle sauce and toss again to combine.
To serve, place some noodles on a plate and then top with meatballs. Serve with rest of the sauce on the side and garnish with green onion and sesame seeds.
