Place a large skillet or wok on the stove and set a plate and two medium bowls nearby so everything is ready once the pan is hot.

Wrap the tofu in a clean kitchen towel or paper towels, set it on a plate, and press it gently for 5 minutes to remove some water.

Cut the green beans, carrot, napa cabbage, and scallions if you have not done that yet. Keep the scallion whites and greens in separate piles.

Put the tofu cubes into one of the bowls and sprinkle 2 tablespoons of the cornstarch over them.

Toss the tofu in the bowl until the cubes are lightly coated on all sides.

In the second bowl, whisk together the soy sauce, rice vinegar, brown sugar, the remaining 1 tablespoon cornstarch, and 1/3 cup water until smooth.

Set the skillet over medium-high heat and let it heat for about 1 minute.

Add 2 tablespoons of the oil to the hot skillet and swirl it to coat the bottom.

Add the tofu to the skillet in a single layer. Leave it alone for about 3 minutes, until the bottom looks golden.

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

Transfer the browned tofu from the skillet to the plate and set it aside.

Add the remaining 1 tablespoon oil to the same skillet.