Rub 1 teaspoon salt on to the mutton pieces. Set aside.
Heat oil over medium-low heat in a heavy bottom pot. When the oil begins to shimmer, add cumin seeds, and briefly fry until fragrant. Fold in chopped onions, and fry on medium-low heat until soft and lightly brown, about 5 to 7 minutes.
Add crushed ginger and garlic, and bird’s eye chili. Fry for roughly 30 seconds or until they no longer smell raw.
Stir in red chili powder, coriander powder, and turmeric powder, and fry for roughly 1 minute until fragrant. Deglaze the pot with a splash of water if needed.
Increase heat to high, and add the mutton. Fry on high heat until no longer pink, and evenly brown, about 3 to 5 minutes.
Bring heat to medium, and add diced tomatoes, and the remainder ½ teaspoon of the salt. Cover, and let the tomatoes soften for about 5 minutes.
Remove lid, increase heat to medium-high, and fry tomatoes on high heat, stirring frequently, until they begin to soften and break down, about 3 to 4 minutes. Keep stirring until the masala separates from the oil.
Add enough water to just submerge the meat (exact quantity of water will depend on the pot), and cover with a tight fitting lid. Bring to a boil, and lower to a gentle simmer, and let the curry cook on medium-low heat, covered, for 1 to 1 ½ hour, depending on when the meat is fork tender. Stir occasionally, and add hot water ¼ cup at a time as needed to prevent the curry from sticking to the pot.
Lift cover, check for salt, and season as needed. Fold in chopped spinach in batches. Partially cover the pot, and let it simmer on low heat for another 20 to 30 minutes, stirring occasionally to make sure it’s not sticking to the pot.
Check for salt, and adjust as needed. If there is still excess liquid, remove lid and increase heat to high to boil out some of the water while stirring frequently.
Lightly rub kasuri methi with your hands, and fold it in the curry. Let it simmer for a few additional minutes.
