Warm the pot. Heat 2 tablespoons olive oil in a large pot over medium heat.
Sauté the vegetables. Add diced onion, carrots, and celery.
Cook, stirring occasionally, until softened and lightly golden, about 6–8 minutes. Season with a pinch of salt.
Build the flavor base. Stir in garlic, tomato paste, cumin, smoked paprika, oregano, and red pepper flakes (if using). Cook for 1–2 minutes until fragrant and the tomato paste darkens slightly.
Add lentils and tomatoes. Stir in the rinsed lentils and crushed or diced tomatoes so everything is coated in the spices.
Pour in broth and simmer. Add 6 cups broth and the bay leaf.
Bring to a boil, then reduce to a gentle simmer. Cook uncovered for 25–30 minutes, or until lentils are tender but not mushy.
Adjust texture. For a creamier body, use an immersion blender to pulse the soup a few times, or scoop out 1 cup, mash it with a fork, and stir it back in. Keep most of the lentils whole.
Add greens. Stir in spinach or kale.
Simmer 2–3 minutes until wilted and tender. Remove the bay leaf.
Brighten it up. Stir in the juice of 1 lemon. Taste and season with more salt and pepper as needed.
Finish and serve. Ladle into bowls and top with chopped parsley.
Add a drizzle of extra-virgin olive oil, a sprinkle of Parmesan or feta, and extra lemon wedges on the side.