If the rice is not already warm, heat it now in a covered saucepan over low heat or in the microwave. Keep it covered so it stays warm for serving.

Wrap the tofu in a clean kitchen towel or paper towels. Set it on a plate and place a small skillet or a few cans on top to press out extra water for 10 minutes.

While the tofu presses, thinly slice the onion, finely chop the ginger, trim the snow peas if needed, and slice the scallions. Keep the scallions separate so they are ready for finishing.

Cut the pressed tofu into 1-inch cubes. Put the cubes into a medium bowl.

Sprinkle 1 tablespoon of the cornstarch over the tofu in the bowl. Gently toss until the tofu looks lightly coated.

In a small bowl, stir together the soy sauce, rice vinegar, sugar, the remaining 1 tablespoon cornstarch, and 1/2 cup water until smooth. Set this sauce near the stove.

Brown the tofu: Heat a large nonstick skillet or wok over medium-high heat for about 1 minute. Add 1 tablespoon of the oil and swirl it across the pan.

Put the tofu into the hot skillet in a single layer. Cook without moving it for 3 minutes, or until the bottom looks golden.

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 browned. Transfer the tofu to a plate.

Build the stir-fry: Add the remaining 2 tablespoons oil to the same skillet. Add the onion and cook over medium-high heat for 2 minutes, stirring often, until it starts to soften.

Add the ginger to the skillet. Stir for 30 seconds, just until fragrant.

Add the cherry tomatoes to the skillet. Cook for 3 to 4 minutes, stirring now and then, until many of the tomatoes split and their juices coat the bottom of the pan.