Heat Peppercorns, Mustard, Cumin, Coriander, And Fenugreek Seeds, Cloves, And Cinnamon In A 12" Skillet Over Medium-High Heat, And Cook, Swirling Pan Occasionally, Until Lightly Toasted, About 2 Minutes.
Transfer To A Bowl, And Let Cool; Working In Batches, Transfer Spices To A Spice Grinder And Process Until Finely Ground.
Transfer To A Small Food Processor Along With Paprika, Vinegar, Turmeric, Sugar, 1⁄4 Of The Garlic, And Half The Ginger; Puree Until Smooth.
Transfer To A Large Bowl, And Add Chicken; Rub Chicken With Spice Mixture.
Cover And Refrigerate At Least 4 Hours Or Up To Overnight. Heat Oil In A 6-Qt. Saucepan Over Medium-High Heat.
Add Onions, And Cook, Stirring, Until Caramelized, About 25 Minutes. Add Remaining Garlic And Ginger Along With Chiles, And Cook, Stirring, Until Soft, About 5 Minutes.
Add Chicken Along With Any Paste In Bowl, Potatoes, And 2 Cups Water, And Bring To A Boil; Reduce Heat To Medium-Low, And Cook, Covered And Stirring Occasionally, Until Chicken Is Cooked Through, About 25 Minutes. Remove From Heat, And Season With Salt; Serve With Rice.
