Set a large nonstick skillet or a well-seasoned wok on the stove. Put a plate and a medium bowl nearby, and warm the cooked rice now if you plan to serve the stir-fry over rice.

Pat the tofu dry again with paper towels so it will brown instead of steam. Put the tofu cubes in a medium bowl.

Add the soy sauce, rice vinegar, maple syrup, and 1 tablespoon of the cornstarch to a small bowl. Add 3 tablespoons of water and whisk until smooth.

Sprinkle the remaining 2 tablespoons cornstarch over the tofu in the bowl. Toss gently until the tofu has a thin, even coating.

Pour 2 tablespoons of the oil into the skillet. Heat it over medium-high heat until the oil looks shiny and moves easily across the pan.

Add the tofu to the hot skillet in a single layer. Cook it without moving it for 3 to 4 minutes, until the bottoms look golden and a little crisp.

Turn the tofu pieces with tongs or a thin spatula. Cook for 3 to 4 minutes more, turning once or twice, until several sides are browned.

Transfer the browned tofu to the plate. Keep the skillet on medium-high heat.

Add the remaining 1 tablespoon oil to the skillet. Put the shiitake mushrooms into the skillet and cook for 2 minutes, stirring once or twice, until they start to soften.

Add the asparagus and red bell pepper to the skillet. Stir-fry for 3 minutes, until the asparagus turns bright green and the pepper starts to soften at the edges.

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

Whisk the sauce again so the cornstarch is fully mixed in. Pour the sauce into the skillet.