Preheat oven to 450 degrees. Prepare one large baking dish or two smaller baking dishes with cooking spray.
Season the chicken breasts with generous pinches of salt and pepper. In a large bowl, toss the zucchini, squash and onion with olive oil until evenly coated. Season with salt and pepper.
If using one baking dish, place all of the vegetables on the bottom of the dish in an even layer. Lay the four chicken breasts evenly on top, then cover each chicken breast with the hummus so that the entire breast is covered. Squeeze the juice of one lemon over the chicken and vegetables. Then sprinkle the pan with smoked paprika or sumac. Thinly slice the remaining lemon, and place the slices in between the chicken and vegetables if desired.
**If using two pans, use one for the chicken and one for the vegetables.**
Bake for about 25-30 minutes, until the chicken is cooked through and the vegetables are tender. Serve immediately.