Preheat the oven to 350 degrees F. Grease a 12-by-18-by-1-inch sheet pan with olive oil.
Heat 2 tablespoons of olive oil in a large (12-inch) saute pan over medium heat. Add the pancetta and saute for 5 minutes, until it begins to brown. Add the leeks and saute for 5 minutes, until tender. Add the zucchini and thyme and saute for 5 to 6 minutes, stirring occasionally, until the zucchini is tender.
Meanwhile, in a large bowl, whisk together the eggs, half-and-half, 1 1/2 teaspoons salt and 1 teaspoon pepper. Stir in two-thirds of the Gruyere and the basil.
Pour the zucchini mixture in an even, well distributed layer into the prepared sheet pan. Pour the egg mixture over the top and sprinkle with the remaining Gruyere. Shake the pan to evenly distribute the egg mixture. Bake for 30 to 35 minutes until puffed and just set in the center. Allow to sit at room temperature for 5 minutes. Cut the frittata into 30 equal size squares, sprinkle with salt, garnish with the julienned basil and serve hot, warm or at room temperature.