Heat a large Dutch oven over medium heat and add the ghee when the pan is hot.
Toss in the onions and cook, stirring frequently, until softened and translucent, about 10 to 15 minutes. (If you have extra time, cook the onions for 30 minutes or until golden brown to develop even more flavor.)
Add the green pepper, ginger, and garlic. Cook, stirring, until fragrant—about a minute.
Toss the chicken with 1 teaspoon Diamond Crystal kosher salt. Add the chicken to the pot.
Cook, stirring frequently, until the chicken is no longer pink, about 5 to 8 minutes. (If you’re using chicken breast, the cooking time will be closer to 3 to 5 minutes.)
Stir in the coriander, cumin, 1 teaspoon of Diamond Crystal kosher salt, 1 teaspoon of garam masala, turmeric, and ground Kashmiri chili powder. Cook, stirring well to distribute all the spices, for about a minute.
Pour in the tomato purée and ½ cup of water and stir to combine.
Increase the heat to bring the sauce to a boil. Then, cover the pot and decrease the heat to maintain a simmer. Cook for 10 to 15 minutes or until the chicken is tender. (Using chicken breasts? The cooking time will be around 5 minutes.)
Stir the chopped spinach into the pot and mix well. Cover the pot and simmer for about 3 to 5 minutes or until the spinach is tender and absorbs all the seasoning.
Stir in the remaining teaspoon of garam masala and add the lemon juice. Taste and adjust the seasoning as needed. Enjoy!