Rinse the red lentils thoroughly under cold running water using a colander.

Set the rinsed lentils aside.

Heat vegetable oil in a skillet over medium heat.

Add finely chopped onions and sliced carrots to the skillet.

Cook, stirring occasionally, until the onions and carrots are softened, about 5 minutes.

Add minced garlic, turmeric, cumin seeds, salt, and peppercorns to the skillet.

Cook, stirring continuously, for 1 minute to release the aromas.

Add the canned tomatoes with their juice to the skillet.

Bring the mixture to a boil, breaking up the tomatoes with the back of a spoon.

Stir in the reserved red lentils and vegetable stock.

Transfer the entire mixture to a slow cooker stoneware insert.

Cover the slow cooker and cook on Low for 8 to 10 hours or on High for 4 to 5 hours, until the carrots are tender and the mixture is bubbling.

Stir in the coconut milk, lemon juice, and finely chopped chili pepper.

Cook on High for 20 to 30 minutes, until the soup is heated through.

When ready to serve, ladle the soup into bowls.

Top each bowl with thin slices of lemon and finely chopped cilantro, if desired.