Grate the zucchini (or squash), sprinkle with salt and leave to drain in a colander for 1 hour. In a bowl, mix together the zucchini, the potatoes, the cheeses, the corn flour, the spearmint and the parsley and season with black pepper.
Beat the eggs with the condensed milk and incorporate into the zucchini and potato mixture. Grease a baking dish with the butter-and-oil mix. Pour in the entire mixture or divide into individual greased baking dishes.
Beat the remaining butter-and-oil mix with the egg yolk and brush the surface of the mixture. Bake for 55΄ - 60΄at approximately 180° - 200° C, until golden brown. Garnish with pine nuts and fresh aromatic herbs and serve.