In a 6- to 8-qt. pot, heat oil over medium-high heat. Add fenugreek seeds; sizzle 2 seconds, then stir in onion and mushrooms and cook, stirring often, until lightly browned, 5 to 7 minutes. Meanwhile, mince ginger and garlic.
Add ginger, garlic, turmeric, cayenne, coriander, cumin, and salt to pot; sizzle a few seconds. Add chicken and cook, stirring often, 3 minutes, or until no longer pink on the outside. Add tomatoes and simmer, covered, 8 minutes.
Stir in spinach and simmer, covered, until wilted, about 3 minutes. Meanwhile, chop enough cilantro leaves and tender stems to yield 1 cup. Stir into pot, along with lemon juice. Serve with cooked rice.