Toss top sirloin with cornstarch, soy sauce, and rice wine. Let sit at room temperature for 20 minutes.
Combine soy sauce, brown sugar, oyster sauce, and sesame oil. Stir until sugar is dissolved and set aside.
Heat 1 tablespoon oil in a large wok or saute pan over high heat. Add marinated beef and saute until browned, about 3-4 minutes. Remove beef and set aside.
Heat another tablespoon in the same pan over medium heat. Add chopped kabocha squash and saute until browned and caramelized. Add ¼ cup water and bring to a boil. Cover the pan and continue to cook until squash is tender. Remove squash and set aside.
Heat another tablespoon oil in the same pan. Add sliced onion and eggplant and saute until browned. Add ¼ cup water and bring to a boil. Cover the pan and continue to cook until eggplant is tender.
Remove lid and add sliced bell pepper, garlic, Thai chile, and ginger. Saute until bell peppers are tender, about 2 minutes. Add back beef, kabocha squash, and stir fry sauce. Bring to a simmer, tossing to coat. Saute for another minute or until everything is well sauced. Remove from heat and toss in chopped cilantro. Sprinkle sesame seeds on top and serve stir fry with rice.
Notes
You can also use other sirloin cuts, flank steak, or skirt steak for the stir fry.