If you use chicken breasts, they will need to be tenderized first. You can skip this step if you use tenderize, chicken tenders which are naturally tender. To tenderize, place breasts between 2 pieces of plastic wrap or wax paper and gently flatten to 1/2 inch with a rolling pin.
Cut chicken into strips 2 inches x 1/2 inch. Put strips in a bowl with soy sauce and toss to coat. Sprinkle 4 TBS of cornstarch over mixture and coat each piece of chicken, set aside.
Trim the roots off the scallions and cut diagonally into pieces, set aside.
With a peeler remove the lemon rind in thin strips and then cut into fine shreds. If you are in a hurry, you can grate the lemon rind instead.
Squeeze juice from lemon and set aside.
Mix remaining TBS of cornstarch with 1 Tbsp of water or stock to form a thickening paste.
Heat about 2 TBS olive oil in a wok or large (10" or more) frying pan and cook the chicken quickly over high heat. Work in very small batches so that chicken browns lightly without steaming.
Each batch should take 3 to 4 minutes to cook. Keep each cooked batch warm on a plate as you fry the rest of the chicken. Add oil as necessary to finish all the stir-fry.
Add 1 TBS oil to skillet. Heat quickly, then add garlic and stir-fry for about 1 minute until fragrant.
Add lemon zest, lemon juice, balsamic vinegar and stock.
Bring to a boil.
Add cornstarch paste and whisk to make a thickened sauce.
Add chicken strips and scallions. Make sure all pieces are coated nicely.
Taste, add salt or pepper if desired.
Continue to heat for a minute or two more. Serve immediately with brown rice.