Step 1

Whisk 1/4 cup water, soy sauce, tomato paste, vinegar, sugar, crushed red pepper to taste and 1 teaspoon cornstarch in a small bowl. Set aside. Cut tofu into 1/2- to 3/4-inch cubes and pat dry. Toss the tofu in a bowl with the remaining 2 tablespoons cornstarch to coat.

Step 2

Heat 1 tablespoon oil in a wok or large skillet over medium-high heat. Add the tofu and spread out across the surface of the pan. Let cook undisturbed for 2 minutes. Gently turn and stir. Continue cooking, stirring occasionally, until lightly browned and crispy, 2 to 3 minutes more. Transfer to a plate.

Step 3

Reduce heat to medium. Add the remaining 1 tablespoon oil to the pan. Add green beans, garlic and ginger; cook, stirring constantly, for 1 minute. Add the remaining 1/4 cup water, cover and cook until the beans are crisp-tender, 2 to 4 minutes. Stir the reserved soy sauce mixture and pour it over the green beans. Cook, stirring, until thickened, about 1 minute. Add the tofu and cook, stirring, until heated through, about 1 minute more.