Heat oil in a medium-large soup pot or Dutch oven over medium. Add the onion and cook for 4-5 minutes or until soft and translucent.
Add the garlic, ginger and curry powder. Cook, stirring frequently, until fragrant, about 2 minutes.
Add the broth, crushed tomatoes, coconut milk, lentils, diced carrot, roasted red pepper, potato, ¼ cup of the cilantro (set aside the remaining ¼ cup), salt and pepper. Bring the mixture to a boil; reduce heat and simmer gently (uncovered), stirring occasionally, until lentils are soft and veggies are tender, about 20 minutes.
Add the remaining ¼ cup cilantro and stir to combine.
If you prefer a thinner soup, you can add a bit more broth at this point.
To serve, divide soup among bowls. Garnish with fresh herbs, diced avocado, thinly sliced scallions and/or with lime wedges.
Soup (without toppings) can be made 3 days ahead. Let cool; cover and chill. Warm gently on the stovetop. If the soup is too thick, you can thin it with a little more broth.