Preheat the oven to 350°F. Grease a 9 x 9-inch baking dish.

In a large bowl, combine spinach, ricotta, milk, parsley, oregano, garlic powder. Add 8 ounces of crumbled feta. Taste and season with salt and pepper. If it seems very dry, add a tablespoon or two of additional milk. Set aside.

In a medium sized, heavy-bottomed saucepan, melt butter and garlic over medium heat. Once garlic is softened slightly (but not browned at all), sprinkle with arrowroot powder and stir to combine. Add half and half or heavy cream and stir to eliminate lumps. Allow it to thicken, then turn off the heat and add cheese, nutmeg, salt, and pepper. Stir well to combine.

Gather all ingredients - spinach filling, white sauce, matzo sheets, toppings, and greased baking dish. Begin to layer ingredients starting with ½ of the white sauce, then 1 full sheet of matzo, ½ of the spinach, another sheet of matzo, the remaining white sauce, the remaining spinach filling, then sprinkle with the remaining 4 ounces of crumbled feta and 1 cup of shredded Parmigiano Reggiano.

Bake for 30-35 minutes, until lightly browned and heated through. Let cool for 5-10 minutes before cutting and serving.