Set a large nonstick skillet or wok over medium heat so it is ready when the tofu is prepared.

Put the tofu on a clean towel or paper towels and press gently to remove extra moisture. This helps it brown instead of steam.

Transfer the tofu to a medium bowl. Sprinkle 1 teaspoon of the cornstarch over the tofu and toss gently so the pieces are lightly coated.

Put the soy sauce, rice vinegar, brown sugar, water, and remaining 1 teaspoon cornstarch in a small bowl. Stir until the cornstarch dissolves and the sauce looks smooth.

Cut and separate the scallions if you have not already, keeping the white parts in one pile and the green parts in another. Keep the green parts for the end.

Add 1 tablespoon of the oil to the hot skillet and raise the heat to medium-high.

Put the tofu into the skillet in a single layer. Let it cook without moving for 3 to 4 minutes, until the bottom looks deep golden.

Turn the tofu pieces with tongs or a thin spatula. Cook for 3 to 4 minutes more, turning as needed, until several sides are golden and the tofu feels firmer. Transfer the tofu to a plate.

Add the remaining 1 tablespoon oil to the same skillet. Put the scallion whites, red onion, and carrots into the skillet.

Cook the vegetables for 2 minutes, stirring often, until the onion starts to soften and the carrots are hot but still a little firm.

Add the celery and water chestnuts to the skillet. Cook for 2 minutes, stirring often, until the celery turns bright and just starts to soften while the water chestnuts stay crisp.

Add the garlic and ginger to the skillet. Stir for about 30 seconds, just until they smell fragrant.