Preheat the oven to 375°. Grease a deep casserole dish with butter.
Peel and slice the potatoes. Put them in a bowl of salted water to prevent them from browning and to aid in extracting some starch while preparing the cream sauce.
Heat a skillet over medium heat. Add the olive oil and butter. Add the onions and thyme. Sauté until the onions are softened, but do not overbrown them.
Add the heavy cream and heat while stirring until just warmed. Be careful not to boil the cream. Season with salt and pepper. Set aside while assembling the dish.
Drain the potatoes in a colander and shake to remove excess water. Layer one-quarter of the sliced potatoes on the bottom of the casserole dish.
Use one-quarter of the cheese and sprinkle it over the potatoes. Repeat the layers of potatoes and cheese three more times.
Pour the cream mixture evenly over the potatoes and cheese. Bake in the oven for about 70 minutes until the potatoes are tender and the sauce is bubbling. Test the potatoes with a sharp knife to ensure they are cooked.
Prep: 30min
Cook: 70min
Total: 110min
