In large saucepan, warm oil over medium-high heat. Add onions and cook, stirring, until lightly browned, about 10 minutes. Add cumin, coriander, turmeric, garlic and ginger; stir to combine.
Add chicken drumsticks and raise heat to high. Cook, turning chicken, until browned on all sides, about 5 minutes. Add cayenne pepper and tomatoes with juices. Bring to simmer.
Cover pan, reduce heat to medium-low and simmer gently for 20 minutes. Increase heat to medium-high and add chopped spinach. Cook, stirring, until tender, about 3 minutes