In a small dry pan over medium-low heat, toast the cumin, coriander, peppercorns, cloves, and cinnamon stick for 2 minutes until fragrant. Do not burn.
Transfer toasted spices to a blender with Kashmiri chilies, garlic, ginger, and vinegar. Blend until a smooth, thick, vibrant red paste forms.
Pour the paste over the 900g of chicken thighs in a bowl. Coat completely and marinate in the fridge for at least 1 hour, preferably overnight.
Heat oil in a heavy pot over medium-high. Add chicken in batches, leaving excess marinade behind. Sear for 3-4 minutes until lightly browned, then set aside.
Add chopped onions to the same pot. Sauté over medium heat for 10-12 minutes until deep golden brown and sweet.
Return chicken to the pot with remaining marinade, tomato paste, sugar, salt, and water. Stir well, cover, and simmer on low for 30 minutes until chicken is tender and oil separates.