1. Combine the tomatoes with their juices, 2 tablespoons vinegar, flour, tomato paste, 1 teaspoon garam masala, ginger, garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper in the carafe of a blender or in a food processor. Puree until well blended and only slightly chunky. Set aside.
2. Spread the potatoes in the bottom of a 5-to-6-quart slow cooker. Sprinkle the chicken with the remaining 1 teaspoon garam masala, 1/4 teaspoon salt and 1/4 teaspoon pepper and lay over the potatoes in a single layer. Pour the sauce over the chicken. Cook on high for 3 1/2 to 4 hours or low for 6 to 7 hours.
3. Uncover the slow cooker and stir in the baby spinach until wilted, about 1 minute. Let the curry stand, uncovered, for 10 to 15 minutes to allow the sauce to thicken a bit. Add the remaining 1 teaspoon vinegar and chopped coriander and season with additional salt and pepper. Serve with additional coriander for garnish, with bread on the side for scooping up the sauce, chutney and/or Greek yoghurt.