To prepare a spinach kugel, you will first need to prepare the spinach. You can buy fresh spinach at your local grocery store, or you can purchase frozen spinach from the frozen section of the store. If you want to make your own spinach, cut the stalks off the stems and chop them into small cubes. Add the cubed spinach to the bowl of your food processor and pulse it until it's finely chopped. Then, add the egg yolks, milk, salt, and pepper. Pulse the ingredients until they're well combined. Pour the kneading mixture into the floured mixture and mix it with your hands until the mixture resembles coarse breadcrumbs. Cover the dough with plastic wrap and refrigerate it for at least 30 minutes.