- Preheat your oven to 375°F (190°C). 
- In a large skillet over medium heat, melt the butter with olive oil. Add the onion and cook until soft and golden, about 10 minutes. 
- Stir in the garlic and cook for another 1-2 minutes until fragrant. 
- In a large mixing bowl, combine the cream, milk, salt, pepper, and 1 cup of the Parmesan cheese. 
- Layer half of the sliced potatoes in a buttered 9x13-inch baking dish. 
- Spoon half of the onion-garlic mixture over the potatoes and pour half the cream mixture on top. 
- Repeat the layers with the remaining potatoes, onion mixture, and cream. 
- Sprinkle the top with the remaining 1/2 cup of Parmesan cheese. 
- Cover the dish with foil and bake for 45 minutes. Remove foil and bake an additional 30 minutes or until the top is golden brown and bubbly. 
- Let it rest for 10 minutes before serving. Garnish with fresh thyme if using.