Line a plate with paper towels. Put the tofu on the plate, cover it with more paper towels, and set a second plate or a small skillet on top. Let the tofu press for 10 minutes to remove extra water.

While the tofu presses, put the soy sauce, white vinegar, and brown sugar in a small bowl. Whisk until the sugar is mostly dissolved.

Trim the green beans. Thinly slice the bell pepper and onion. Thinly slice the garlic and scallions, and keep the dark green scallion tops separate for serving.

Uncover the tofu and pat it dry again. Put the tofu cubes in a medium bowl, sprinkle the cornstarch over them, and toss gently until the pieces have a light, even coating.

Set a large nonstick skillet or well-seasoned wok over medium-high heat. Add 2 tablespoons of the oil and heat it until it looks shiny.

Put the tofu in the skillet in a single layer. Cook it without moving it for 3 to 4 minutes, until the bottoms look lightly browned.

Turn the tofu pieces with tongs or a thin spatula. Cook for 4 to 5 minutes more, turning once or twice, until several sides are golden and the coating looks crisp.

Transfer the tofu to a clean plate. Keep the skillet on the stove.

Add the remaining 1 tablespoon oil to the skillet. Put in the onion, garlic, and the white and light green scallion pieces. Cook for 1 to 2 minutes, stirring often, until the onion starts to soften and the garlic smells fragrant.

Add the green beans to the skillet. Cook for 3 minutes, stirring every so often, until they turn bright green and pick up a few browned spots.

Add the bell pepper to the skillet. Cook for 2 to 3 minutes, stirring often, until the pepper is just starting to soften but still has some bite.

Lower the heat to medium. Pour the soy-vinegar sauce into the skillet and stir for about 30 seconds, scraping the bottom of the pan, until the sauce starts to bubble.