Combine the vinegar, flour, tomato paste, 1 teaspoon each garam masala and curry powder, garlic, tomatoes with their juices, ginger, 1/2 teaspoon salt and 1/4 teaspoon pepper in a blender or food processor. Puree until well blended and only slightly chunky.
Spread the potatoes in the bottom of a 5 1/2- to 6-quart slow cooker. Sprinkle the chicken with the remaining 1 teaspoon each garam masala and curry powder and a generous amount of salt and pepper. Lay the chicken over the potatoes in a single layer. Pour the sauce over the chicken. Cook on high for 4 hours or 6 hours on low.
Uncover the slow cooker and stir in the baby spinach until wilted. Add the cilantro and season with salt and pepper.
Serve over rice with an additional sprinkle of garam masala and cilantro for garnish, with bread on the side for scooping up the sauce, some chutney and Greek yogurt.
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