These simple-to-make squares are a wonderful way to enjoy that versatile and delicious green vegetable, spinach. Try them alongside simple roast chicken or pork. Light yet satisfying, they make great party hors d'oeuvres. Serve hot or at room temperature.
Position a rack in the middle of the oven and preheat to 350 degrees F. Butter an 8 x 11-inch baking dish and set aside.
Heat the oil in a large skillet over medium heat. Add the onion and cook, stirring frequently, until the onion is soft, 6 to 8 minutes. Add the garlic and cook until fragrant, about 1 minute. Add the spinach (it may be necessary to do this in batches) and raise the heat to medium-high. Cook, stirring frequently, until the spinach wilts. Remove the pan from the heat.
Whisk the eggs and milk together in a large bowl. Stir in the bread crumbs, parsley, dill, lemon zest, salt, pepper, and cayenne. Add the spinach mixture and the Parmesan, and stir to combine. Transfer the mixture to the prepared pan and bake until the top has browned and set, about 30 minutes. Let cool for at least 10 minutes before cutting into squares.