Echo Farm's Au Gratin Potatoes with Cheddar-Stout Sauce

Ingredients

• 1/2 teaspoon freshly ground black pepper
• 1 teaspoon chopped fresh thyme
• 1/2 cup dark beer, such as stout or porter
• 3 tablespoons King Arthur Unbleached All-Purpose Flour
• 3 medium baking potatoes (about 2 lbs.), peeled and thinly sliced
• 1 medium onion, finely chopped
• 1 teaspoon salt
• 3 tablespoons Cabot Unsalted Butter
• 8 ounces Cabot Sharp Cheddar, shredded (about 2 cups)
• 1 1/2 cups milk