• Caramelize the onions: Melt 3 tablespoons butter with olive oil in a large skillet over medium heat. Add sliced onions, salt, and optional brown sugar. Cook for 30–40 minutes, stirring occasionally, until deeply golden and caramelized. Add a splash of water as needed to prevent sticking. Stir in thyme and remove from heat.
• Prepare the oven: Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
• Make the cream sauce: Melt 3 tablespoons butter in a saucepan over medium heat. Add garlic and cook for 30 seconds. Whisk in flour and cook for 1 minute. Slowly whisk in milk and heavy cream until smooth.
• Add the flavor: Whisk in Better Than Bouillon Roasted Beef Base, Dijon mustard, black pepper, and optional nutmeg. Simmer for 3–5 minutes until slightly thickened. Remove from heat and stir in ½ cup Gruyère cheese.
• Layer the potatoes: Arrange half the sliced potatoes in the prepared baking dish. Top with half the caramelized onions, half the cream sauce, and ½ cup Gruyère.
• Repeat: Add the remaining sliced potatoes, caramelized onions, and cream sauce. Sprinkle with the remaining ½ cup Gruyère and Parmesan cheese.
• Bake: Cover tightly with foil and bake for 60 minutes. Remove foil and bake for another 25–35 minutes, until the potatoes are completely tender and the top is golden and bubbly.
• Rest: Allow potatoes to rest for 15 minutes before serving. Garnish with fresh thyme.

Recipe Notes

• Use a mandoline for evenly sliced potatoes.
• Caramelize onions ahead of time to save on prep.
• Always taste the cream sauce before adding additional salt.
• Freshly shredded Gruyère will give you the smoothest, creamiest sauce.
• Baking time can vary depending on the thickness of your potato slices and the depth of your casserole dish.
• Reserve a few caramelized onions for topping if desired.
• Refrigerate leftovers in an airtight container for up to 3–4 days.