1. Preheat the oven to 350 degrees . To make the pesto, pulse garlic, arugula, peas, pine nuts, cup Parmesan, salt and pepper in a food processor until combined, about 15 seconds. With the processor running, slowly add olive oil and process until pureed, adding water if necessary to thin out pesto.
2. In a large bowl, toss together pesto, ziti and 1/2 cup water. Spread half the ziti mixture in a 9-by-13-inch casserole dish and top with half the mozzarella. Pour 1/2 cup water over casserole, add remaining ziti mixture and top with remaining mozzarella. Pour 1 cup water over casserole, cover tightly with aluminum foil and bake 1 hour.
3. Remove foil and turn on the broiler. Sprinkle remaining Parmesan over casserole and broil 5 minutes, or until cheese is brown and bubbling.