Prepare the Sauce: In a small bowl, mix together the soy sauce, oyster sauce (if using), ketchup, brown sugar or honey, rice vinegar, and water or chicken broth. Stir until smooth, then set aside.

Sear the Beef: In a large skillet or wok, warm one tablespoon of oil over medium-high heat until it’s hot and ready for cooking. Add the thinly sliced beef and stir-fry for 2-3 minutes, or until it’s just browned on the outside.

Sauté the Vegetables: Add the remaining oil to the skillet. Add the onion, red bell pepper, and green bell pepper, and stir-fry for 2-3 minutes, until they start to soften.

Add Aromatics and Pineapple: Add the minced garlic, grated ginger, and pineapple cubes to the skillet. Stir-fry for an additional 1-2 minutes, until fragrant and the pineapple begins to caramelize slightly.

Combine and Add Sauce: Return the beef to the skillet, then pour in the prepared sauce. Stir everything together and let it cook for 1-2 minutes.

Thicken the Sauce: Add the cornstarch mixture to the skillet, stirring well to combine. Allow the sauce to cook for another minute or until it has thickened to your liking.

Serve: Serve the stir-fry hot over steamed rice, garnishing with green onions or sesame seeds if desired.