Dry-roast the coriander seeds, fennel seeds, nigella seeds, fenugreek seeds, cumin seeds, mustard seeds, and dried red chilies in a pan over medium heat for 2-3 minutes until fragrant. Cool and grind to a coarse powder — this is your homemade achari masala.
In a large bowl, whisk the yogurt with ginger garlic paste, turmeric, salt, and 2 tablespoons of the achari masala. Add the chicken pieces and coat well. Cover and marinate for 30 minutes (or up to 2 hours refrigerated).
Heat the mustard oil in a large kadai until it just starts to smoke, then reduce the heat. Add the nigella seeds and fennel seeds and let them sizzle for 30 seconds.
Add the sliced onions and cook over medium-high heat for 8-10 minutes until golden brown. Stir in the ginger garlic paste and cook for 1 minute, then add the tomato puree, red chili powder, turmeric, coriander powder, and salt. Cook until the oil separates, about 5 minutes.
Add the marinated chicken with all the marinade and the water. Bring to a boil, then reduce the heat, cover, and simmer for 25-30 minutes until the chicken is tender and the gravy thickens.
Stir in the slit green chilies and garnish with fresh coriander. Serve hot with naan, paratha, or steamed rice.