Heat oven to 375°F. Lightly coat 8-inch square or round baking pan with nonstick cooking spray. In large nonstick skillet over medium heat, heat oil. Add onion. Sauté until slightly softened, 3 minutes. Add zucchini. Increase heat to medium-high and sauté until zucchini is soft and all liquid has evaporated, 7 to 10 minutes. Stir in 1/4 teaspoon of the salt. Remove from heat.
In large bowl, beat eggs, milk, and remaining 1/4 teaspoon salt. Add carrots, zucchini, cheese, and dill. Spread in prepared pan.
Bake until quiche is just set in center, 45 minutes. Transfer to wire rack. Let cool at least 10 minutes before cutting. Serve warm or at room temperature.