Baked Cheddar Spinach Squares

frozen spinach, thawed 1 cup flour 1 teaspoon baking soda 1/2 teaspoon kosher salt 1 cup milk (non-fat is fine to use) 2 eggs 1/2 cup onion, finely chopped 3/4 pound (3 cups) shredded sharp cheddar cheese, divided (see note)