1. Preheat oven to 425°F (220°C).
2. Wash and dry potatoes. Cut a thin slice off the bottom of each potato, just enough so they don’t roll. Cut thin slits across the potatoes with a sharp knife, taking care not to cut all the way through. Brush potatoes generously with oil, and sprinkle with salt, pepper, and paprika. Place on a lightly greased baking sheet and roast for about an hour, basting every 15 minutes and separating the slices slightly with a knife to get some of the oil and spices between the slits.
3. Meanwhile, in a deep frying pan, sauté the onions in oil until soft and golden brown. Add the rest of the sauce ingredients except for the pastrami and bring to a boil. Once the sauce is boiling, lower heat, and simmer for about 20-25 minutes, stirring occasionally to prevent burning. Leave the pot uncovered so the liquid can evaporate and the sauce will reduce and thicken. When done, add the pastrami and cook for an additional 15 minutes.
4. Remove potatoes from oven and top each one with several heaping spoons of the meat sauce. Return to oven for an additional 15 minutes to set.
To slice the potato perfectly, take two wooden spoons or two chopsticks and place them on either side the potato, one on either side, so you don’t accidentally cut too far.