SAUTE onions in 1 Tbsp of canola oil for 4-5 minutes, or until browned. Stirring so they don’t burn.
ADD vinegar and COOK until liquid evaporates.
TRANSFER mixture to food processor or blender. ADD remaining oil, and BLEND.
PLACE onion mixture in a 4-quart slow cooker.
PLACE ginger, garlic, chili peppers, turmeric, coriander, garam masala, and cinnamon in the food processor or blender. BLEND into a paste, stopping to scrape sides of the bowl. ADD to the slow cooker.
With a mortar and pestle or spice/coffee grinder if you have one that is used just for spices, CRUSH the peppercorns, salt, and mustard seed into a coarse powder. ADD mixture to slow cooker.
PLACE cut chicken potatoes (if using) and ¼ to ½ cup water in the slow cooker. STIR everything to combine.
COOK on low and cook for 6 hours, stirring an hour or so before serving. TASTE for seasoning ( I usually add a little more salt, adding about ½ teaspoon salt at a time, stirring, and tasting until I get it just right)