Arrange half of the potatoes in the bottom of a buttered 2.5-quart shallow casserole dish. Make the buttermilk-blue cheese sauce as follows: melt butter in a medium saucepan over low heat. Add garlic and shallots and saute for 1 minute. Add flour, stir, and cook for 3 minutes more. Pour in milk and buttermilk all at once. Cook, stirring constantly, until sauce thickens and bubbles. Remove from heat and stir in white pepper, mayonnaise, mustard, Worcestershire, parsley and half of the blue cheese. Layer all the mushrooms over the potatoes in the casserole; top with half of the sauce and distribute the grated jack cheese over the sauce. Layer remaining potatoes over cheese and spoon remaining sauce on top. Sprinkle with remaining blue cheese. Bake at 180ºC for 45 minutes. Sprinkle potato chips over top and bake an additional 10 minutes. Meanwhile, blanch the broccoli. Arrange broccoli around outer edges of casserole and garnish with pimento; bake 5 minutes longer.