1. Rinse the lentils in a fine mesh sieve until the water runs clear.
2. Heat the oil in a medium saucepan over medium-high heat. Add the onion and salt and cook, stirring, until soft and translucent, about 5 minutes. Add the garlic, ginger, cumin, and cayenne and cook until fragrant, 1 minute. Stir in the tomatoes, three of the lemon slices (or lemon juice), the stock, and the lentils.
3. Bring the mixture to a boil, then reduce the heat to medium-low, cover, and simmer for 20 to 30 minutes until the lentils are tender.
4. Squeeze the remaining lemons over the stew and season with salt to taste. Serve the stew over the sesame rice and sprinkle it with the garnish, if using.
Rice:
1. Rinse the rice under cold running water in a fine mesh sieve until the water runs clear. Combine the rice, water, and salt in a small saucepan and bring to a boil over medium-high heat. Reduce the heat to low, cover, and cook for 45 to 60 minutes or until the rice is tender and all the water has been absorbed. Remove the rice from the heat and allow it to rest for 10 minutes, then fluff with a fork.
2. While the rice cooks, toast the sesame seeds over medium heat in a small skillet, stirring constantly, for about 5 minutes until lightly golden. Fold the sesame seeds into the rice.