Soak the beef cubes in a bowl of cold water for 1 hour to remove excess blood water (change water once halfway through). Drain and pat thoroughly dry with paper towels.
In a mixing bowl, combine the dried beef cubes with cooking wine, oyster sauce, light soy sauce, 3 teaspoons of black pepper, and sugar. Massage by hand and marinate for 30 minutes.
While beef marinates, prepare vegetables: cut onion into squares, slice peppers into sections, and halve the garlic cloves.
Heat a heavy skillet or wok over medium-high heat. Add butter. Once melted and bubbling, arrange beef cubes in a single layer. Sear for about 1 minute, flip, and cook for another minute until browned. Remove beef and set aside.
Reduce heat to low. In the remaining fat, slowly fry the garlic cloves until they are golden brown and soft.
Increase heat to high. Add the onion chunks and stir-fry for 1 minute until fragrant.
Add the cooked beef and sliced peppers back into the pan. Season with salt and the remaining 1 teaspoon of coarse black pepper.
Toss everything rapidly on high heat for 2 minutes to blend flavors. Serve immediately with steamed rice.