1. Pour 3 tablespoons olive oil into a 10- to 12-inch frying pan over medium heat. Add onion and stir often until soft, 6 to 8 minutes. Add zucchini and 1 teaspoon salt; cook, stirring occasionally, until zucchini is soft and most of the liquid has evaporated, about 10 minutes longer.
2. Meanwhile, in a 5- to 6-quart pan, bring 4 quarts water to a boil. Add penne and cook until tender to bite, 10 to 12 minutes. Drain.
3. Coat halibut with remaining 2 tablespoons olive oil and sprinkle all over with salt and pepper. Place in a 10- to 12-inch nonstick frying pan over high heat and cook, turning once, until browned on both sides, about 4 minutes total. Pour in wine, reduce heat to maintain a simmer, and cook until fish flakes easily with a fork, about 5 minutes longer. Break halibut into flakes.
4. Stir lemon peel into zucchini mixture. In a large bowl, combine pasta and zucchini mixture. Add flaked halibut and liquid in pan and sprinkle with all but 2 tablespoons mint; mix to coat pasta. Sprinkle with remaining mint.