Heat the oven to 375°F and grease a 9-by-13-inch dish. Brown the beef and onion in a skillet and drain.
Add the zucchini and cook for 7 minutes until tender and dry, then add garlic for 30 seconds. Stir in orzo, broth, milk, salt and pepper.
Bring to a simmer, pour into the dish and cover tightly with foil.
Bake for 20 minutes. Uncover, stir in the pesto and 1 cup mozzarella, then top with remaining mozzarella and Parmesan.
Bake uncovered for 10 to 15 minutes until the orzo is tender and top browned. Rest 10 minutes.