Heat the oil in a large saucepan over a medium heat and cook the onions for 10 minutes, stirring occasionally.
Add the garlic, carrot, ginger, ground coriander, cumin, turmeric and chili and cook, stirring, for 1 minute. Add the potato and lentils and stir to coat with the spice mixture.
Add the tomatoes, coconut milk, stock, garam masala, salt and sugar, bring to the boil and cover with a lid.
Reduce the heat to medium and simmer, covered, for 20 minutes, stirring occasionally.
While the curry simmers, cook the rice.
Add the peas to the curry and simmer, uncovered, for 5 minutes.
Remove from the heat and stir in the coriander and lemon juice.
Serve on a bed of rice