Set grill temperature to 350˚F.
In a large shallow dish, combine zucchini, squash, and onion.
Sprinkle with salt and pepper. Drizzle with olive oil and lemon juice.
Spray grates with cooking spray and grill vegetables, with grill lid closed, turning occasionally, for 6 to 8 minutes or until tender.
Remove from grill and sprinkle with rosemary and feta cheese.
Serve warm or at room temperature.