Preheat oven to 300 degrees F. Butter a casserole dish.
In a large frying pan over medium to medium-low heat, melt the butter with the olive oil. Add the sliced onions and gently fry the until soft and lightly golden, approximately 20 minutes. Remove from heat and spread 1/2 the sauteed onion in the prepared casserole dish.
Add a layer of sliced potatoes and season with salt, pepper, and thyme leaves. Add the rest of the onions, potatoes, salt, pepper, and thyme leave. Pour the beef stock over the top.
Cover the dish tightly with aluminum foil and bake until all the liquid has been absorbed and potatoes are tender when pierced with a fork, approximately 2 to 3 hours. Remove the aluminum foil for the last 1 hour of cooking if you like a crisp top. Remove from oven and let stand 10 minutes.
Before serving, sprinkle additional fresh thyme leaves over the top.
Makes 4 servings.
* Learn how easy it is to make outstanding homemade Beef Stock.
Potatoes Slow-Cooked In Beef Stock Recipe: https://whatscookingamerica.net/KarenCalanchini/PotatoesSlow-Cooked.htm