Heat the olive oil in a large pot or Dutch oven over medium heat. Add the onion, carrot, celery, and bell pepper. Cook, stirring occasionally, until the onion softens and becomes translucent, about 7 minutes.

Stir in the garlic, curry powder, and red pepper flakes. Cook for 1 minute, stirring constantly, until fragrant.

Add the rinsed lentils and canned tomatoes with their juice. Cook for 1 minute, stirring often, to deepen the flavors.

Pour in the vegetable broth and add the salt and pepper. Increase the heat to bring the soup to a boil, then reduce to a gentle simmer. Cover and cook for 30 minutes, or until the lentils are tender.

Using an immersion blender, give the soup 2 or 3 quick pulses to thicken it slightly. Alternatively, let the soup cool a bit, then transfer 2 cups to a blender. Secure the lid, cover with a tea towel to protect from steam, and blend until smooth. Return the puree to the pot.

Remove the pot from the heat. Stir in the chopped cilantro and the juice of half the lemon. Taste and adjust seasoning, adding more salt, pepper, or a pinch of red pepper flakes for extra heat.

Serve the soup in bowls, garnished with fresh cilantro leaves and lemon wedges from the remaining lemon half. Pair with crusty sourdough bread for dipping. Enjoy!