Set up the pan: Set a large nonstick skillet or wok on the stove and place a plate near it for the cooked tofu.

Put the tofu cubes in a medium bowl. Sprinkle the cornstarch over the tofu and toss gently so the cubes are lightly coated on all sides.

In a small bowl, stir together the soy sauce, brown sugar, the juice from half the lime, and 2 tablespoons water until the sugar is mostly dissolved.

Check that the shallot is sliced, the chopped lemongrass is ready, the mushrooms are sliced, the snap peas are trimmed, and the scallions are sliced so everything can go into the skillet quickly.

Heat the skillet over medium-high heat. Add 2 tablespoons of the neutral oil and swirl to coat the bottom.

Add the tofu to the hot skillet in one layer. Let it cook without moving it for 3 minutes so the first side can brown.

Turn the tofu pieces with tongs or a spatula. Cook for 3 to 4 minutes more, turning once or twice, until several sides are golden and the coating looks crisp.

Transfer the browned tofu to the plate.

Add the remaining 1 tablespoon oil to the same skillet. Add the sliced shallot and chopped lemongrass to the skillet and cook for 1 minute, stirring often, until fragrant and slightly softened.

Add the sliced mushrooms to the skillet. Spread them out and cook for 2 minutes without stirring much so they can start to brown.

Stir the mushrooms and keep cooking for 3 to 4 minutes, until they have shrunk and the skillet looks drier.

Add the snap peas to the skillet. Stir and cook for 2 minutes, until they turn bright green but still look crisp.