• Preheat the oven to 350° F.
• Melt together the butter and onions in a large skillet over medium-high heat. Cook, stirring occasionally until softened, about 10 minutes. At this point, you'll want to add the wine and continue to cook another 5 minutes until the onions are deeply caramelized.
• Add the garlic and thyme, then season with salt and pepper. Cook another 2 minutes, then remove from the heat.
• Butter a 9x13 inch casserole dish. In a large glass measuring cup, combine the cream, milk, a pinch of cayenne, salt, and pepper. Then add the Gruyere cheese. Pour 1/3 of the milk mix into the bottom of the baking dish. Layer in 1/3 of the potatoes, and 1/2 of the onions. Add another 1/3 milk mix, then 1/3 of the potatoes, and then the remaining onions. Add the remaining potatoes, then pour over the last of the remaining milk mix. Try to get the potatoes in an even layer.
• Cover and bake 1 hour 45 minutes to 2 hours. Remove the foil and top with cheddar cheese. Bake another 20 minutes, until the cheese is golden and melty. Serve the dish warm, topped with fresh thyme.

Notes:

• To make Ahead: prepare and bake through step 5 but stop cooking after 2 hours. Let chill, then cover, and keep in the fridge for up to 4 days. Before baking, bring the dish to room temperature, add the cheddar cheese, and bake another 20-30 minutes.