Preheat oven to 350°F. Butter a 1 1/2 to 2-quart baking dish.
Boil potatoes with skin on and 1 bay leaf for about 20 minutes. Peel and cut potatoes into cubes.
Over medium-high heat, melt 8 Tablespoons of butter. Add the onions and cook, stirring occasionally, until the onions start to soften, 3 to 4 minutes. Sprinkle the flour over the onions and stir to incorporate. Cook for a minute or 2 but don’t let it get too dark. Whisk in the milk, getting out all the lumps. Add the broth (whisk again if there are still lumps). Bring to a simmer and allow it to thicken, about 3-5 minutes. Season with salt and pepper. Turn off the heat and stir in the Aged Cheddar and sour cream. Stir in potato cubes.
Cover with foil and bake in preheated oven 30 minutes. Mix breadcrumbs with 3 tablespoons butter and 1/2 cup Aged Cheddar. Remove foil and top potatoes with breadcrumb mixture another 15 minutes.
Optional: Top with a sprinkle of paprika or canned fried onions.