Ingredients
Prep Time: 30 min
Total Time: 2 hr
3 tablespoons + 1 teaspoon butter, divided
3 tablespoons flour
1 teaspoon dry mustard
1 teaspoon kosher salt
2 cups milk
1/2 teaspoon freshly ground black pepper
1 tablespoon chopped fresh thyme leaves
2 pounds yukon gold potatoes, peeled and thinly sliced crosswise
1 medium onion, diced
Preparation
