Prep your ingredients. Dice the onion, celery, carrots, and bell pepper.
Mince the garlic. Rinse the lentils in a fine-mesh strainer and pick out any debris.
Sauté the aromatics. Heat 2 tablespoons of olive oil in a large pot over medium heat. Add onion, celery, and carrots with a pinch of salt.
Cook 6–8 minutes, stirring occasionally, until softened. Add bell pepper and cook 2 minutes more.
Bloom the spices. Stir in garlic, cumin, smoked paprika, thyme, and red pepper flakes. Cook 30–60 seconds until fragrant, stirring so nothing burns.
Build the base. Add tomato paste and cook for 1 minute to caramelize slightly.
Pour in crushed tomatoes and stir to combine.
Add lentils and broth. Tip in the rinsed lentils, bay leaf, and vegetable broth. Stir, bring to a gentle boil, then reduce to a steady simmer.
Simmer until tender. Cover partially and cook 25–35 minutes, stirring now and then, until lentils are tender but not mushy. Add more broth or water if the stew gets too thick.
Add the greens. Stir in spinach or kale.
Cook 2–4 minutes until wilted and vibrant.
Finish and season. Remove the bay leaf. Stir in lemon juice, then taste and adjust with salt and black pepper. For extra body, mash a small scoop of lentils against the pot with a spoon and stir back in.
Serve. Ladle into bowls and garnish with chopped parsley or cilantro.
Drizzle with olive oil or add a dollop of yogurt if you like.