IN a medium bowl, combine the yogurt, garlic, & 1/4tsp of the dill weed. Add the chicken & let marinate 20 minutes.
Heat a 12" nonstick skillet over moderate heat. Add the chicken & marinade & cook, stirring, for 5 minutes or until chicken is no longer pink.
Using a slotted spoon, transfer the chicken to a plate & keep warm. Spoon 1/4C of the marinade into a small bowl; stir in the cornstarch & salt until dissolved. Wipe out the skillet, discarding any remaining marinade.
Add the oil to the skillet & heat over moderate heat. Add the onion, squash, & remaining 1/2tsp of dill weed. Cover and cook, stirring occasionally, for 8mins or until the veggies are crisp tender. Add the sugar snap peas and tomatoes & cook 3 minutes more until the peas are tender crisp.
Stir in the chicken & cornstarch mixture, bring the liquid to a boil and cook for 1 minute or until the sauce has thickened and the chicken is heated through. Serve warm with steamed rice, or serve chilled as a main-dish salad.