Potatoes Au Gratin

4-5 large Idaho russet potatoes, peeled and sliced 1/8 th inch thick
1/4 cup butter
1/2 sweet yellow onion, diced
1 tbsp flour
Sea salt and fresh cracked pepper to taste
2 cups of non fat milk
2 cups of extra sharp cheddar cheese, shredded (divided)----I used Mozzerella
1/4 dry Italian bread crumbs

Preheat oven to 375 degrees. Spray a baking dish with cooking spray. Scrub potatoes and peel. Cut into 1/8-inch slices to measure about 4 cups. If you have a mandolin slicer, use it.

Melt butter in saucepan over medium heat. Cook onion in butter about 2 minutes, stirring occasionally, until tender. Stir in flour, salt and pepper. Cook, stirring constantly, until bubbly; remove from heat.

Slowly stir in milk and 1 1/2 cups of the cheese. Heat to boiling, stirring constantly. Boil and stir 1 minute until the cheese is completely melted.

Spread potatoes in casserole dish. Pour cheese sauce over potatoes. Bake uncovered for 1 hour. Mix remaining cheese and the bread crumbs; sprinkle over potatoes; add a bit more salt and pepper. Bake uncovered 15 to 20 minutes longer or until top is brown and bubbly.