Here's a quick and easy casserole based on the Greek classic spanakopita that replaces phyllo dough with matzoh crackers. If you like, you can stir 1/4 teaspoon grated nutmeg and 3 tablespoons chopped fresh dill into the spinach mixture for a richer flavor.
Preheat the oven to 375 degrees. Butter or oil an 8-by-8-inch baking dish.
Combine the spinach, onion, and ¼ teaspoon of salt in a large skillet and cook over medium heat, stirring frequently, until the onion is tender and most of the liquid has evaporated from spinach, 6-7 minutes. Transfer to a medium bowl and let cool slightly. Stir in about 2/3 of the feta. In a blender, combine the milk, cottage cheese, eggs, and remaining ½ teaspoon salt. Blend until smooth.
Arrange 2 matzohs in the bottom of the prepared baking dish and top with 1/2 of the spinach mixture. Top with 2 more matzohs, remaining spinach, and remaining 2 matzohs. Pour the milk mixture over the top and sprinkle with the remaining feta. Cover the dish with foil and bake for 30 minutes. Remove the foil and continue to bake until top is browned and egg is set, 15-20 minutes longer. Cool 15 minutes before serving.