Preheat oven to 425 degrees. Grease a 13 x 9-inch baking dish, or prepare 6-8 ramekins for individual servings.

Cook bacon in a medium saucepan over medium-low heat until crisp, about 10-13 minutes. Using a slotted spoon remove the bacon from the pan onto paper towel. Reserve the bacon drippings in the skillet.

Turn the temperature up to medium heat. Add the onions to the bacon drippings along with ¼ teaspoon of salt. Cook, stirring frequently until onions start to become soft and caramelized, about 25 minutes. Add 1 tablespoon of water to the skillet if the onions start to become too dark too quickly.

Transfer onion to a large bowl. Return the saucepan to the hot burner.

Pour the broth into the saucepan. Using a wooden spoon, scrape up any browned bits that have formed on the bottom of the pan. Bring broth to a simmer.

Add the bacon, thyme, remaining salt, pepper and potatoes to the bowl with the onions. Stir until mixture is well-combined. Spread into your baking dish or spoon an even amount into your gratin dishes. Carefully pour the hot broth over top.

Dot the surface of the casserole with butter.

Bake uncovered for 45-55 minutes until the potatoes are fork tender and the edges are golden brown. Remove from oven and allow to sit for 20 minutes before serving.