Heat olive oil in a large pot or Dutch oven over medium heat. Add the onion, carrot, celery, and bell pepper, and cook, stirring often, until the onion is softened and translucent, about 7 minutes.

Add the garlic, curry powder, and chili flakes. Cook, stirring constantly, until fragrant, about 1 minute.

Stir in the red lentils and canned tomatoes, and cook for 1 minute, stirring often, to deepen the flavors.

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

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

Remove the pot from the heat and stir in the chopped cilantro and the juice of half the lemon. Taste and adjust seasoning with more salt or chili flakes if desired.

Serve the soup garnished with fresh cilantro leaves, alongside crusty bread and lemon wedges from the remaining half lemon. Add extra lemon juice for brightness if you like.