Whisk 1/2 cup each flour and cornstarch, 1/2 teaspoon kosher salt, 1 cup water and 1 egg in a bowl. Combine 2 cups breadcrumbs and 1/2 cup grated pecorino in another bowl. Slice 2 large zucchini 1/4 inch thick; dip in the batter, then dredge in the breadcrumb mixture. Fry in 1/2 inch of 350 degrees Fvegetable oil until golden, 3 minutes per side. Drain on paper towels and season with salt and pepper. Serve with mayonnaise mixed with chopped parsley and lemon juice.