1. Preheat your oven to 425 degrees Fahrenheit or 220 degrees Celsius.

2. Peel and slice the leek and yellow onions, then set them aside for later. Next, peel the potatoes and slice them into rounds about 1 centimeter or half an inch thick.

3. Place the sliced potatoes in a large pot and pour in enough broth to cover them completely. Bring the mixture to a boil, then reduce the heat and let it simmer until the potatoes are almost tender, about 10 to 12 minutes. Add the sliced leek and onions during the last 2 to 3 minutes of cooking.

4. Drain the potatoes and transfer them to a greased 23x33 centimeter or 9x13 inch baking dish.

5. Now it's time to make the garlic cream sauce. Peel and finely chop the garlic, then combine it in a bowl with the crème fraiche, grated hard cheese, and grated fresh cheese. Season with salt and pepper to taste.

6. Pour the garlic cream sauce over the potatoes in the baking dish. Transfer the dish to the middle rack of the oven and bake for 15 to 20 minutes, or until the top is golden brown and the sauce is bubbly.