Preheat oven to 400°F. In 13" X 9" baking dish, combine lemon zest, 1 tablespoon of the oil, and onion, reserving 1 tablespoon. Spread in even layer. Roast until onion is tender, stirring occasionally, about 15 minutes.
Remove from oven. Stir in zucchini, squash, and garlic. Return to oven and roast 10 minutes. Remove from oven.
Increase oven temperature to 450°F. Push vegetables to one side and add fish. Spoon vegetables over fish.
Return to oven and roast until fish is opaque in center, 8 to 10 minutes for thin fillets and 12 to 15 minutes for thicker fillets.
Meanwhile, combine remaining tablespoon oil with vinegar, water, mint leaves, and remaining onion. Spoon over bass fillets and serve.