Preheat the oven to 350ºF. Grease a shallow 2-½-quart casserole. Arrange half of the potato slices in the bottom of the prepared dish and set aside. In a medium saucepan over low heat, melt the butter. Add the garlic and shallots and saute for 1 minute. Add the flour, stir or whisk to blend, and cook for 3 minutes more. Add the milk. Cook, stirring constantly, until the sauce thickens and bubbles. Remove the saucepan from the heat. Add the pepper, mayonnaise, mustard, Worcestershire sauce, parsley, and half of the blue cheese and stir to incorporate. Add the buttermilk and stir. Layer the mushrooms over the potatoes in the casserole. Spread half of the sauce on top, then sprinkle with the Monterey Jack cheese. Layer the remaining potatoes over the cheese and cover with the remaining sauce. Sprinkle with the remaining blue cheese. Bake for 45 minutes. Distribute the potato chips on top and bake for 10 minutes more. In a saucepan over medium heat, steam or boil the broccoli until crisp-tender. Arrange the broccoli around the outer edge of the casserole and garnish with pimiento. Bake for 5 minutes more.