Heat the olive oil in a large pot over medium heat. Add the onion and cook for 4 minutes until it softens.
Stir in the garlic, ginger, cumin, turmeric, and coriander. Cook for 1 minute until the spices smell fragrant.
Add the cauliflower florets, red lentils, diced tomatoes with their juice, and vegetable broth. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 20 minutes until the lentils are soft and the cauliflower is tender.
Use an immersion blender to partially blend the soup, leaving some chunks for texture. Stir well.
Ladle into bowls and top with chopped cilantro. Serve with lemon wedges on the side.