Wash, drain and finely chop spinach. Place it in a large pot. Cook over medium heat until the water it releases evaporates, about 5 minutes. Remove from heat, set aside to cool. Squeeze dry the spinach to get rid of excess moisture.
In a separate pot, melt butter over medium heat. Add flour and using a whisk, continuously stir the flour-butter mixture for about 3-4 minutes. Slowly add milk while stirring the mixture with a whisk to avoid lumps. Add ¾ of the grated cheese, stir. Add spinach, stir with a spoon. Continue to cook until the mixture is hot, stirring frequently. Add salt and pepper to taste.Remove from heat, place in a serving dish. Sprinkle remaining cheese on top, serve hot.