Heat the oil in a large pot or Dutch oven over medium heat.
Sauté the diced onion for 3 to 5 minutes, until it becomes soft and translucent.
Stir in the minced garlic and diced carrots, and cook for another 3 minutes until the garlic is fragrant.
Add the water, rinsed red lentils, and ground cumin. Increase the heat to bring the soup to a boil.
Once boiling, reduce the heat to low, cover the pot, and let it simmer for 20 to 25 minutes. The soup is ready when the lentils have softened and started to disintegrate, thickening the soup.
In a small bowl, place the miso and salt. Ladle a small amount of the warm soup liquid into the bowl and stir vigorously until the miso dissolves into a smooth, lump-free paste.
Pour the miso mixture back into the soup pot and stir to combine.
Serve immediately.