Heat oil in a large skillet over medium high heat.
Generously season the meat with salt and pepper. Working in batches, add the meat to the hot skillet and brown on all sides but still a little red on the inside, about three minutes. Remove meat from pan and set aside.
Reduce heat to medium. Add carrot, zucchini, and squash to pan. Season with salt and pepper. Cover the skillet with a lid and cook until vegetables are crisp-tender, about five minutes.
Whisk together soy sauce, vinegar, garlic, ginger and cornstarch. Pour mixture over vegetables and simmer until the sauce begins to thicken, about 3-5 minutes.
Return meat and any accumulated juices to the pan. Stir to coat the meat with the sauce.
Sprinkle with sesame seeds, if desired. Serve over rice.