To make the marinade, combine 2 teaspoons of the cornstarch and soy sauce in a bowl and mix well. Add the beef and stir to coat evenly. Let stand for 10 minutes.

To make the sauce, combine the orange juice, oyster sauce, honey and remaining 2 teaspoons cornstarch in a small bowl and stir until honey dissolves. Set aside.

In a small frying pan, toast the sesame seeds over medium heat, shaking the pan frequently, until lightly colored, 3 to 4 minutes. Immediately pour onto a plate to cool.

Place a stir-fry pan over high heat until hot. Add the oil, swirling to coat on all sides. Add the ginger and cook, stirring, until fragrant, about 10 seconds. Add the beef and stir-fry until caramelized on the edges and still slightly pink in the center, 2 to 3 minutes. Add the sauce and cook, stirring, until the sauce boils and thickens slightly, about 20 seconds. Stir in the orange segments and pineapple chunks and cook for 30 seconds to heat through.

Transfer to a serving plate, sprinkle with the sesame seeds and serve.