Heat oil in a large pan over medium high heat. Add the garlic, onion, tomatoes, cumin and coriander, and cook for a minute.
Add the carrots, salt, pepper, and cayenne, cook until the carrots are slightly soft - about 5 minutes.
Add the cooked lentils, tahini paste, and lemon juice. Cook, stirring, for a couple of minutes. Turn down the heat to medium and cook gently, stirring, for a few minutes more, until hot and thickened.
Spread out the lentils on a flat platter and scatter on the sliced onion and cilantro.