Cut the chicken breasts into bite-size pieces and place them in a medium bowl. Season with salt, pepper, and the 1/2 teaspoon garlic powder, then sprinkle the 3 tablespoons cornstarch over the chicken and toss until evenly coated.
In a small bowl, whisk together the sauce: 1/3 cup water, 1/3 cup lemon juice, lemon zest, 1/4 cup granulated sugar, and 1 tablespoon cornstarch until smooth.
Heat 2 tablespoons olive oil in a large skillet over medium-high heat until hot.
Add half of the coated chicken in a single layer without crowding; cook for about 4 minutes, then flip and cook another 3–4 minutes until browned and cooked through (165°F).
Transfer the cooked chicken to a plate and add the remaining 1 tablespoon olive oil to the skillet. Cook the second half of the chicken the same way until browned and cooked through.
Return all the cooked chicken to the skillet, remove the pan from the heat, then pour in the prepared sauce. Place the skillet back on the heat and simmer for 30 seconds to 1 minute, stirring, until the sauce thickens and coats the chicken.
Toss the chicken in the sauce to coat thoroughly and serve topped with chopped scallions and lemon slices, if desired.