Preheat the oven to 375 degrees F. Melt the butter in a large pot over medium-high heat. Add the onions and saute until softened and beginning to brown. Add the garlic and continue cooking 1 minute longer. Stir in the cream, broth and thyme, and season with the salt and pepper. Add the potatoes and bring to a simmer. Continue cooking until the potatoes are fork-tender, 10 to 15 minutes.
Transfer the potatoes and cream to a shallow baking dish. Sprinkle the cheeses evenly over the potatoes. Bake until bubbly and the surface begins to brown, 15 to 20 minutes. Let sit 10 minutes before serving.
Recipe courtesy of Kelsey Nixon