Preheat the oven to 350℉. Spray a 9x13-inch baking dish with nonstick spray.
In a deep, large skillet over medium-high heat, melt the butter. Add the onion; cook, stirring often, until the onions soften (4 to 5 minutes). Sprinkle the flour over the onions; cook and stir for 2 minutes. Whisk the milk into the onion/flour mixture. Whisk in the Dijon, then whisk in the broth. Let the mixture simmer for 3 to 4 minutes. Season with salt and pepper. Remove the pan from heat. Stir in the cheese and sour cream. Add the potatoes and stir. Transfer to the prepared pan.
Sprinkle the Ritz crumbs over the potatoes. Sprinkle bacon on top. Then sprinkle on the Parmesan cheese.
Cover with foil; bake for 30 minutes. Then remove the foil and bake for 15 more minutes, until bubbling. Let the potatoes sit at room temperature for a little while before serving.