Get the pan ready: Set a large nonstick skillet or well-seasoned cast-iron skillet on the stove, and place a plate next to it for the browned tofu.

Wrap the tofu in a clean kitchen towel or paper towels, set it on a plate, and press it for 5 minutes with a heavy skillet or a few cans.

Mix the sauce: In a small bowl, whisk together the soy sauce, 1/2 cup water, brown sugar, 1 tablespoon of the cornstarch, and the black pepper until smooth.

Prep the vegetables: Slice the bell peppers, onion, and mushrooms if they are not already prepared. Slice the garlic, and keep the scallion whites separate from the green tops.

Unwrap the tofu and cut it into bite-size rectangles or squares about 1/2 inch thick.

Put the tofu in a medium bowl, sprinkle the remaining 1 tablespoon cornstarch over it, and toss gently so the pieces are lightly coated.

Brown the tofu: Heat the skillet over medium-high heat for about 1 minute. Add 1 tablespoon of the oil and swirl it across the surface.

Put the tofu into the hot skillet in a single layer. Cook it without moving it for 4 to 5 minutes, until the bottoms are golden brown.

Turn the tofu pieces with tongs or a thin spatula. Cook the second side for 3 to 4 minutes, until it is browned, then transfer the tofu to the plate.

Cook the vegetables: Add the remaining 1 tablespoon oil to the skillet. Put the onion and bell peppers into the skillet and cook for 3 minutes, stirring a few times, until they start to soften but still hold their shape.

Add the mushrooms to the skillet. Cook for 4 minutes, stirring often, until the mushrooms shrink and most of the liquid in the pan cooks away.

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