Chop the zucchini, squash and onion; cut the zucchini and squash in half length-wise then half each of the halves again length-wise; chop the spears into rough 1 inch pieces
Cut the onions into similar size pieces; toss all the vegetables together
In large casserole dish, lay chicken in a flat single layer; season with salt, pepper, and garlic powder
Layer the vegetables over the chicken; spread over the chicken evenly; dollop the pesto over the vegetables and top with the cheeses
Bake on middle rack for 35-45 minutes until the chicken is cooked through with an internal temperature of 165F; if the cheese is getting too brown cover loosely with foil