Sprinkle zucchini and summer squash slices with salt and arrange them in a colander to drain for 1 to 2 hours. Rinse in cold water and pat dry and place in a large mixing bowl. Butter a 10-inch deep-dish pie plate and preheat the oven to 350°F.
In a blender, add cream, eggs, herbs, starch, pepper and nutmeg and blend until smooth. Pour liquid into the bowl with zucchini and summer squash and toss. Mix in grated Manchego cheese. Then layer the zucchini and summer squash in the pie plate, leaving a ⅛-inch gap at edge. Pour in the remaining liquid. Top with grated Parmesan and bake for 45 to 60 minutes, until slightly golden on top and a toothpick comes out clean. Let rest for 10 minutes before slicing.