Preheat the oven to 350 degrees.
Remove the moisture from the spinach. To do this easily and thoroughly, my favorite trick is to wrap a clean tea towel around it and, literally, wring it out. (If you have never done this, you will be amazed at how much water is in there!) Then, separate the spinach with two forks or with your fingers.
In a large bowl, mix the flour, baking soda, salt, milk and eggs with a fork. Stir in the onion, two cups of the cheese, and the spinach.
Spread the mixture evenly in a greased, 12×8 baking dish. (Two 9×5 loaf pans have a similar surface area and you could eat one now and freeze one for later.) Sprinkle with the remaining one cup of cheese, and bake for 30 to 35 minutes or until cooked through and golden brown on top. (Sometimes, I move the dish to the top rack for the final two minutes or so to brown the top a little more. Just watch closely if you do this, so as not to burn.)
