Spinach Souffle

I haven't posted in a while but I have been cooking a lot and I knew I needed to post this souffle. I've always heard that making the perfect souffle is a little difficult but this was so easy and delicious! I made this one morning for Michael and I for breakfast, it was is not the quickest meal to make but definitely worth the time. Since this was my first souffle, I used Mark Bittman's Cheese and Spinach Souffle recipe from, How to Cook Everything. It was very simple to follow, my only recommendation is make sure you have a large enough bowl when adding the whipped egg whites to the mixture otherwise it will overflow.

1/2 stick butter
1/4 cup all purpose flour
1 and 1/2 cups milk, warmed to the touch
6 eggs, separated
salt and pepper to taste
1/2 teaspoon dry mustard
1/2 cup mozzarella (the recipe called for Parmesan but I only had mozzarella and cheddar on hand)
1/2 cheddar