Rinse the lentils thoroughly under cold water, picking out any debris or shriveled ones, and set aside to drain.
Dice the carrot, celery, and onion into 1/2-inch pieces. Mince the garlic and juice the lemons.
Heat olive oil in a large Dutch oven over medium heat. Add the onion with a pinch of salt and cook, stirring occasionally, for 8-10 minutes until soft and translucent.
Add the garlic and cook for 1-2 minutes until fragrant. Add the carrots and celery and cook for 4-5 minutes until they begin to soften.
Push vegetables to the sides, add tomato paste to the center, and cook for 1 minute before stirring in. Add cumin, oregano, turmeric, coriander, and red pepper flakes; stir for 1 minute until fragrant.
Add fire-roasted tomatoes, rinsed lentils, vegetable broth, and bay leaves. Season with salt and pepper, and bring to a boil.
Reduce heat to medium-low, cover with the lid slightly ajar, and simmer for 28-35 minutes until lentils are tender.
Remove bay leaves. Use an immersion blender to partially blend about one-third of the soup for a thicker, creamier consistency (optional).
Stir in the spinach or kale and cook for 2-5 minutes until wilted. Remove from heat and stir in the fresh lemon juice.
Taste and adjust seasoning with salt, pepper, or additional lemon juice. Serve hot, drizzled with olive oil and garnished with parsley and feta cheese.