Fill a large saucepan fitted with a steamer attachment with water until it is about one-third full. Place the saucepan on high heat, without the steamer attached, and bring the water to a rapid boil.
While the water is heating, prepare the vegetables. Peel the sweet potato and carrots, then chop them into large, evenly sized chunks so they steam at the same rate.
Once the water has reached a rolling boil, reduce the heat to medium-high. Place the sweet potato and carrot into the steamer basket, attach it over the saucepan, and cover with a lid. Steam the vegetables for about 15 minutes, or until they are very soft and easily pierced with a fork.
Drain the lentils in a colander and rinse them under cold water a few times. Transfer the lentils to a small heatproof bowl and place the bowl on top of the vegetables inside the steamer. Cover with a lid and steam for a further 5 minutes.
Using a tea towel, carefully remove the bowl of lentils from the steamer and spoon out the vegetables. Allow to cool for 5 to 10 minutes.
Transfer the steamed carrots and sweet potato to a food processor, then add the lentils. Process on medium-high speed until the mixture reaches a thick mashed potato consistency.
For younger babies, you may prefer to use breast milk to loosen the puree to the right consistency. Alternatively, boiled water works just as well. Add one tablespoon at a time, blending between each addition, until you reach a texture that suits your baby’s age and feeding stage.