Add the cumin and coriander seeds to a dry saucepan and toast for 1-2 minutes. Add the oil and the onions. Cook for 5 minutes, stirring regularly until browned.
Add the garlic, ginger, green chillies and chicken pieces and cook for another 2-3 minutes, stirring regularly.
Add the chili powder, garam masala and turmeric powder along with the lemon juice, tomatoes and 500ml water.
Bring to the boil, mix well, reduce the heat and simmer for 15 minutes, stirring regularly.
Add the lentils and simmer for another 15 minutes, stirring regularly, until cooked.