MICROWAVE flour, butter and milk in large microwavable bowl on HIGH for 6-8 minutes, whisking every 2 minutes, or until sauce is slightly thickened. Stir in blue cheese and salt, until sauce is smooth.
LAYER half the potatoes in a 13 x 9 inch baking dish, sprayed with cooking spray. Top with half the cheese sauce. Top with remaining potatoes and sauce. Cover with foil.
BAKE at 425°F for 30 minutes. Remove foil and bake an additional 45 minutes, or until potatoes are tender.
