Scrub the potatoes and cut them into thin slices and place in a 9x13 baking dish. Cut the onion into rings and place all over the top of potatoes. Set aside.
In a saucepan over medium heat, melt butter then slowly add flour, stirring constantly until it becomes pasty and then forms a ball. Cook for about 1 minute, continuing to stir. Add broth and milk (together) into the flour/butter mix, whisking constantly until creamy and thick (5 - 10 minutes), but do not boil.
Pour the thickened sauce over the potatoes and onions sprinkle lightly with salt and pepper. If you like you can sprinkle on some fresh or dried parsley for color as well.
Place in the oven and bake at 350º for 30-40 minutes until the potatoes are tender and the sauce is lightly browned.
Variations: Add chopped up leftover meat, veggies, etc. to sauce before pouring OR add 1- 2 tsp of Crushed Red Pepper Seeds for a little kick
