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.
The unbaked casserole can be covered and refrigerated for a make-ahead meal. Reheat in the oven before serving.