1. Make the Garlic–Thyme Cream
Melt butter in a saucepan over medium heat. Add onion and cook until soft, sweet, and lightly golden. Add garlic and sauté 30 seconds. Stir in flour and cook 1 minute to form a roux.
2. Infuse & Thicken
Slowly whisk in milk and chicken broth until smooth. Add thyme sprigs and simmer 4–5 minutes until thickened and aromatic. Remove thyme sprigs. Season with 1 tsp salt and ¼ tsp pepper. Off heat, whisk in 1 cup Gruyère until fully melted and silky.
3. Assemble
Butter a baking dish. Layer sliced potatoes, lightly seasoning each layer with remaining salt and pepper. Pour the warm Gruyère–thyme cream evenly over the potatoes. Sprinkle the top with the remaining ½ cup Gruyère.
4. Bake
Convection Oven: Bake at 375°F for 60 minutes (cover first 40 minutes, uncover last 20 minutes).
Standard Oven: Bake at 375°F on the middle rack for 65–70 minutes (cover first 45 minutes, uncover to brown).
5. Rest & Serve
Let rest 10 minutes before serving so the sauce sets. Garnish with fresh thyme and flaky sea salt.