Classic Potatoes Au Gratin Recipe | Say Mmm

Ingredients

• 1/4 cup (1 / 2 stick) butter or margarine
• 1/4 cup flour
• 1 tsp. salt
• 1/2 tsp. dry mustard
• 1/4 tsp. pepper
• 2-1/2 cups milk
• 1 pkg. (8 oz.) KRAFT Shredded Cheddar Cheese , divided
• 2 Tbsp. chopped onion
• 6 cups thinly sliced peeled potatoes (about 6 to 8 medium)