In a saucepan over medium heat, add brown sugar, Blue Dragon Mirin Sauce, dark soy sauce, ginger, honey, garlic and Blue Dragon Sesame Oil. Stir until sugar is dissolved, increase heat and bring to a simmer. In a small bowl, whisk 1 tbsp + 1 tsp of cold water with corn starch. Add corn starch mixture to saucepan and simmer until thickened. Remove from heat and set aside.
Pour 1 tbsp of vegetable oil to a frying pan over high heat. Add onion, broccoli, and mushrooms and stir-fry for 1 minute. Add 3 tbsp of cold water, stir, and let evaporate. Transfer to a plate and set aside.
Heat remaining 1 tbsp of vegetable oil in pan. Sear beef, add reserved vegetables, and stir. Pour in reserved sauce and stir to coat well. Sprinkle servings with green onions and sesame seeds.