Preheat oven to 375 degrees.

Grease a baking dish and set aside. (I used a deep pie dish)

Add butter to a saucepan and add onions and garlic. Place over medium heat and cook, stirring often until onions are translucent. Add flour and salt and whisk together, cooking for about 2 minutes. Add milk, cream and broth and continue whisking until sauce thickens. Remove from heat.

Add a layer of potato slices to cover bottom of dish. Pour about 1/3 of sauce over potatoes and spread evenly.

Mix the shredded cheese together. Sprinkle about 1/3 of cheese over sauce. Continue layering ending with cheese.

Bake for 1 hour 15 minutes to 1 hour 25 minutes or until potatoes are cooked through. Garnish with fresh thyme.

Allow potato gratin to set 15 minutes before serving.