Preparation

• Preheat oven to 350°.
• While the onions caramelize, measure out your remaining ingredients.
• Cut the onions into ¼ inch strings. Melt 2 tbsp. butter and olive oil in a large skillet over medium-low heat. Add the onions and thyme. Let them cook slowly for about 30 minutes, until they’re brown and caramelized.
• Once done, remove and set aside.
• Increase heat to medium. Add 4 tbsp. butter to the pot along with the garlic. Cook for 1 minute. Add flour and whisk to incorporate.
• Gradually add the milk, whisk to incorporate. Bring to a boil, then remove from heat.
• Add mustard powder, salt, pepper, and cayenne. Stir to combine.
• Let the roux cool down a bit. Reserve 1 heaping cup of cheddar cheese and gradually sprinkle the rest in.
• Stir in cream cheese, then add the caramelized onions.
• Peel the potatoes, rinse, and pat dry. Cut them into thin and even slices, about ⅛ inch thick.
• Arrange half of them on the bottom of a lightly greased 9 x 13-inch baking dish and top with half of the cheese sauce. Add remaining potatoes and remaining cheese sauce. Top with remaining cheddar cheese.
• Cover and bake at 350° for 45 minutes. Remove cover and bake for 40 more minutes, until the potatoes are tender.
• Let sit for 5-10 minutes, garnish with fresh parsley, and serve.