Preheat your oven to 350°F (175°C) and lightly grease a 9-inch glass pie pan with cooking spray.
Very thinly slice the zucchini using a mandoline or sharp knife to yield about 4 cups. Set aside the remaining zucchini slices for decoration.
Heat olive oil in a skillet over medium-high heat. Add the leeks, 3 cups of zucchini slices, and 1/8 teaspoon of salt. Cook, stirring often, until softened, about 4-5 minutes. Let cool slightly.
In a medium bowl, whisk together the eggs, half-and-half, 3 tablespoons of chives, and the remaining 1/8 teaspoon of salt.
Layer half of the cooked vegetable mixture into the prepared pie pan. Sprinkle with 1/4 cup feta and 1/2 cup fontina. Repeat with the remaining vegetable mixture and fontina.
Slowly pour the egg mixture evenly over the top. Decoratively layer the reserved zucchini slices on top in a circular pattern, then sprinkle with the remaining 1/4 cup feta.
Place the pie pan on a rimmed baking sheet and bake in the preheated oven for 35-40 minutes, or until puffed and set in the center.
Sprinkle with the remaining tablespoon of chives and let cool slightly before slicing. Serve warm or at room temperature for the best flavor.