Preheat an oven to 400 degrees F (200 degrees C). Combine the potatoes and spinach in a 9x13-inch baking dish; set aside.
Melt 3 tablespoons of butter in a saucepan over medium-low heat. Stir in the onion, and cook until dark brown, about 15 minutes. Stir in the garlic and flour; cook 1 minute more. Season with salt, and stir in the milk and cream. Bring to a simmer over medium heat; cook and stir until the sauce has thickened, about 10 minutes. Stir in 3/4 of the Muenster cheese until melted, and pour over the potatoes.
Cover the potatoes with aluminum foil, and bake in the preheated oven 30 minutes. Uncover, and cook 20 minutes more, then sprinkle with the remaining Muenster cheese, return to the oven, and bake until the potatoes are tender and the cheese is bubbly, about 10 minutes more.
