Heat peppercorns, mustard, cumin, coriander, and fenugreek seeds, cloves, and cinnamon in a pan over medium-high heat, and cook until lightly toasted, about 2-3 minutes. Transfer to a bowl, and let cool, then grind with a pestle and mortar. Mix with the paprika, vinegar, turmeric, sugar, 1⁄4 of the garlic, and half the ginger; puree until smooth. Put in a bowl and rub over the chicken with spice mix. Cover and refrigerate for a few hours to allow spice mix to work its magic.
Heat oil in a saucepan over medium heat. Add onions, and cook until caramelized. Add remaining garlic and ginger along with chiles, and cook until soft. Add chicken with sauce, potatoes, and 2-3 cups water(or as needed), and bring to a boil; reduce heat to low, and simmer gently until chicken is cooked through. Remove from heat, and season with salt; serve with rice and garnish with coriander.
Enjoy. Add some natural yogurt on the side if it's too spicy.