Preheat oven to 350 F.
For the filling, place a large skillet over medium-high heat with EVOO, 2 turns of the pan. Add onion and saut until soft and slightly caramelized, 6-8 minutes. Add garlic and corned beef to the skillet and season with salt and pepper.
Meanwhile, potatoes into a large pot of salted water and bring up to a boil. Let potatoes boil for about 5 minutes, until tender. Drain and let cool.
Once the potatoes are cool enough to handle, cut the top third off and scoop out the guts of the potatoes, using a small spoon or melon baller. Be careful not to puncture through the sides or bottom of the potato. Place the guts into a large mixing bowl and set aside. Discard the extra potato tops.
Mash potato guts in a large bowl with milk, sour cream, mustard, cheese and onion and corned beef mixture. Season with a bit more salt and pepper to taste.
Divide the filling among all the hollowed-out potatoes and place them onto a baking pan. Bake 10-15 minutes, until hot and slightly crispy on top.
Garnish each potato with a little drained sauerkraut or finely sliced red cabbage. Drizzle with homemade Russian dressing.