Heat the olive oil in a large pot or Dutch oven over medium heat. Once shimmering, add the diced onion, carrot, celery, and bell pepper. Cook, stirring often, until the onion softens and turns translucent, about 7 minutes.

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

Add the rinsed red lentils and the can of diced tomatoes (with juice). Cook for 1 minute, stirring often, to enhance their flavors.

Pour in the vegetable 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, cover with a kitchen towel to protect your hands from steam, 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—add more salt if needed, or an extra pinch of red pepper flakes for more heat.

Serve the soup garnished with fresh cilantro leaves, crusty sourdough bread, and lemon wedges from the remaining half lemon. Squeeze extra lemon juice over each bowl for brightness. Enjoy!