Toss the beef and potatoes with the curry powder, 1 teaspoon cumin, 1 tablespoon ginger, the garlic, 1 teaspoon salt and a few grinds of pepper in a 6-quart slow cooker. Crumble 1/2 slice of naan into small pieces over the beef and potatoes, then pour in the tomatoes. Cover and cook on low, 7 hours.
Uncover and skim off any fat from the top of the mixture. Stir well to combine, then let stand 10 minutes. Meanwhile, combine the cilantro, pickled jalapenos and brine, 1/4 cup water and the remaining 1/4 teaspoon cumin and 1 tablespoon ginger in a food processor. Pulse until combined; season with salt and pepper.
Stir 3 tablespoons of the cilantro puree into the slow cooker; season with salt and pepper. Reserve 2 cups beef curry for Beef and Vegetable Handpies. Serve the remaining curry with the naan; top with the remaining cilantro puree.