Before starting to cook, assemble all your ingredients, prepping and measuring as necessary.
With a spoon, scoop out the seeds of each cucumber half. Quarter the halves, thickly slice on the diagonal and set aside.
In a large nonstick skillet, heat the oil over medium heat until hot but not smoking Add the chicken and cook until golden brown about 3 minutes. Using a slotted spoon, transfer the chicken to a plate.
Add the scallions, garlic and ginger, stirring to coat. Add the broth and cook, stirring frequently until the scallions have softened, about 3 minutes. Add the sugar and cook for 1 minute to melt. Add the vinegar and cook for 1 minute, scraping up any browned bits that cling to the bottom of the pan.
Stir in the coriander, salt and red pepper flakes and cook for 1 minute. Add the reserved cucumber and the tomatoes, stirring to coat. Return the chicken to the skillet and simmer just until the chicken is cooked through and the vegetables are crisp-tender, about 3 minutes. Bring to a boil, stir in the mint and cornstarch mixture and cook, stirring constantly, until the mixture is slightly thickened, about 1 minute.
Divide the chicken mixture among 4 plates and serve. Delicious with jasmine rice.
