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Heat the oil in a large pot over medium high heat. Add the mustard seeds and cover the pot while they pop. When the popping slows down, add the cumin seeds and cook until fragrant and darkened, about 30 seconds. Add the onions with a pinch of salt. Let the onions cook, stirring occasionally, for 10 minutes.
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Add the ginger, garlic, turmeric, paprika and tomato paste. Mix into the onions and let them cook--if they seem like they might burn or scorch, add a splash of water. Cook another 3 minutes.
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Add the brown sugar, 2 teaspoons garam masala, zucchini and chicken and mix them into the onions. Mix in the yogurt (or sour cream). Cook for 7 minutes, until the chicken is mostly browned.
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Add the cilantro and spinach and mix into the onions and chicken. Cook for another 5 minutes. When the chicken is cooked through and saucy, add 1 teaspoon of garam masala. Taste for salt and additional garam masala.