Warm the pot: Heat olive oil in a large pot over medium heat. Add onion, carrots, and celery with a pinch of salt. Cook 6–8 minutes, stirring occasionally, until the onions are soft and translucent.
Add aromatics: Stir in garlic and tomato paste.
Cook 1–2 minutes until the tomato paste darkens slightly and smells sweet, not raw.
Toast the spices: Add cumin, smoked paprika, and dried thyme or oregano. Stir for 30 seconds to bloom the spices.
Build the base: Pour in the tomatoes and stir to loosen any bits from the bottom of the pot. Add the lentils, broth, bay leaf, and a pinch of red pepper flakes if using.
Simmer: Bring to a gentle boil, then reduce heat to low and simmer uncovered.
For red lentils, cook about 20–25 minutes. For brown lentils, cook 30–35 minutes. Stir a few times so nothing sticks.
Add greens: Stir in chopped kale or spinach in the last 5 minutes of cooking.
Simmer until wilted and tender.
Season and brighten: Remove the bay leaf. Stir in lemon juice. Taste and adjust with salt and black pepper.
If the soup is too thick, add a splash of water or broth.
Optional texture upgrade: For a creamier feel, use an immersion blender to blend a few quick pulses, or scoop out 1 cup of soup, blend it, and return it to the pot.
Serve: Ladle into bowls and top with chopped parsley. Add a drizzle of olive oil or chili oil if you like a little richness or heat.