Make the sauce. In a small saucepan, combine oyster sauce, soy sauce, sugar, rice vinegar, ketchup, and water. Warm it over low to medium heat, stirring until the sugar dissolves and the sauce becomes smooth. Heat for another minute or two without boiling, then set aside.
Prep the ingredients. Slice the cucumber into sticks, cut the pineapple into chunks, slice the onion into wedges, dice the bell pepper, and cut the tomatoes into small wedges. Mince the garlic and slice the beef into thin pieces.
Cook the beef. Heat oil in a hot wok over medium-high heat. Add the beef and sear until just cooked, then transfer to a plate.
Fry the garlic. Add the garlic to the same wok and cook until fragrant and lightly golden.
Stir fry the vegetables. Add the pineapple, cucumber, onion, and bell pepper. Cook briefly so they soften slightly but still stay crisp.
Add the sauce. Pour in the sweet and sour sauce and toss well so the vegetables absorb the flavor.
Finish with tomatoes and beef. Add the tomatoes, then return the cooked beef to the wok. Toss everything together until coated in sauce. Serve hot with jasmine rice.