Preheat oven to 350°F. In a medium saucepan, combine potatoes, 1 tsp. salt and cold water to cover. Bring to a boil and cook until potatoes are tender when pierced with a fork, about 10 minutes. Drain potatoes well in a colander.
In a 10-inch, ovenproof, nonstick skillet, melt butter over medium heat. Add red onions and cook until soft, about 8 minutes. Add garlic and cook for 2 minutes longer. Add potato, zucchini, mint, remaining 1 tsp. salt and pepper and cook until zucchini is softened, about 5 minutes.
In a medium bowl, beat eggs and vinegar with a fork. Pour egg mixture over vegetables in skillet and stir to blend. Cook undisturbed over medium heat until side begins to set but center is still wet and bubbling, about 3 minutes.
Place skillet in oven and cook until eggs are set, rotating pan once for even cooking, about 10 minutes. To serve, loosen frittata from skillet and cut into wedges from pan. Serve warm or at room temperature.
Frittata may be made up to 4 hours ahead.
This recipe also appeared in the March 2006 issue.