Heat the oil in a large saucepan. Add the onion and cook for 5 minutes until soft.
Add the garlic, ginger, turmeric and cumin. Cook for 1 minute until fragrant.
Add the red lentils and water. Stir well and bring to the boil.
Reduce the heat and simmer for 20 minutes, stirring occasionally, until the lentils have broken down into a thick, creamy mixture.
Stir in the garam masala and season well with salt and a squeeze of lemon juice.
Serve topped with fresh coriander, with rice or naan bread on the side.