Preheat the oven to 400 degrees F.
For the potato topper: Boil the potatoes in salted water until just tender. Drain and transfer to a large bowl. Heat the butter and EVOO in a small skillet over medium heat and toast the caraway seeds. Pour the caraway mixture over the potatoes and season with salt and pepper. Spread the potatoes on a baking sheet and squish them with the bottom of a water glass to flatten them. Roast until crisp, 25 to 30 minutes. Leave the oven on.
Meanwhile, for the casserole filling: Heat the EVOO in a large Dutch oven over medium-high heat. Add the corned beef and cook until browned, and then remove to a plate. Add the ground beef and cook until browned. Add the garlic, celery, bay leaf, carrots and onions and season with salt and pepper. Cook until the vegetables are soft, about 5 minutes. Return the corned beef to the skillet.
Heat the butter in a small skillet over medium heat. Whisk in the flour and cook, whisking, until light golden. Whisk in the beer, Worcestershire, mustard and consomme; season with pepper.
Pour the gravy over the corned beef mixture and stir to combine. Transfer to a casserole dish and top with the sauerkraut, cheeses and potatoes. Bake until the cheese is melted and the casserole is heated through.
Meanwhile, for the Russian dressing topper: Combine the sour cream, ketchup, relish and hot sauce and season with salt and pepper.
Drizzle the casserole with the dressing and garnish with watercress just before serving.
Cook's Note: The unbaked casserole can be covered and refrigerated for a make-ahead meal. Reheat in the oven before serving.