Make the Parsley Pesto: Put the almonds into a food processor and pulse until the finely minced. Add the parsley, oil, Parmesan, salt and puree to combine. Reserve the pesto covered in the refrigerator.
Make the Frittata: Halve the leeks lengthwise and then cut crosswise into 1/2-inch pieces. Put the sliced leeks in a colander and rinse really well under cool water, checking for dirt between the layers. Drain well; you should have about 4 cups of sliced leeks.
Coat a 10-inch skillet with 2 tablespoons of the oil and place over medium heat. When the oil is hot, add the leeks, garlic, and thyme. Cook, stirring occasionally, until the leeks are tender but not brown, about 10 minutes. Season with salt and pepper. Transfer the leek mixture to a 9-by-13-inch baking dish that has been coated with non-stick cooking spray and spread them out in an even layer.
To same skillet, add the remaining 1 tablespoon of oil and return to medium heat. Add the zucchini and sauté for 5 minutes, until tender. Season with salt and pepper. Transfer the zucchini to the baking dish, and spread out on top of the leeks.
Put the eggs, ricotta, and reserved Parsley Pesto in a large bowl and whisk until blended. Season with salt and pepper. Gently stir with a wooden spoon so the egg mixture is fully incorporated with the vegetables.
Preheat the oven to 300-degrees F (150-degrees C).
Put the baking dish into a larger roasting pan and pour hot water into the roasting pan to come halfway up the sides of the baking dish to create a water bath.
Bake the frittata for 50 minutes. Gently shake the pan, and if it still appears loose in the center, return to the oven for another 5 minutes. It should be set and only slightly jiggly in the center.
Remove the frittata from the water bath and let it rest at room temperature before cutting.