Sharp Cheddar Scalloped Potatoes

Ingredients

• 1/4 cup butter, cubed
• 1/3 cup all-purpose flour
• 3/4 teaspoon salt
• 1/2 teaspoon ground mustard
• 1/2 teaspoon white pepper
• 2 cups half-and-half cream
• 1-1/2 cups shredded sharp white cheddar cheese
• 1-1/2 cups shredded sharp yellow cheddar cheese
• 6 cups thinly sliced peeled Yukon Gold potatoes (about 2 pounds)
• 2 small onions, finely chopped