Heat a medium soup pot to medium heat. Add the cumin seeds, mustard seeds, and red chili flakes. Toast the spices for 1 minute, stirring constantly.
Scoop out half of the spices to reserve as a garnish if desired.
Add the onions and allow them to soften for about 3 minutes. If they begin to stick, add a tablespoon of water or broth.
Add the garlic and ginger and stir for 30 seconds.
Add the carrots, lentils, and vegetable broth. Bring the pot to a rolling boil, then lower it to simmer.
Add the plant milk.
Simmer the soup for 15 minutes until the lentils are tender and start to break down. You can leave the pot uncovered or cover with a lid leaving a space for the steam to escape.
Once the lentils are cooked, remove the soup from the heat. Puree the soup using an immersion blender or transfer it to a blend, a few batches at a time, and blend until smooth.
Garnish with the reserved, toasted spices.