Heat olive oil in a large stockpot over medium heat.
Add onion, carrots, and celery; cook until vegetables soften, approximately 7 to 8 minutes.
Add minced garlic, cumin, and smoked paprika, stirring constantly for 1 minute until fragrant.
Pour in the rinsed lentils, diced tomatoes with their juices, and vegetable broth.
Bring the mixture to a boil, then reduce heat to low and cover with a lid.
Simmer for 35 to 40 minutes or until the lentils are tender but not mushy.
Remove from heat and stir in the fresh spinach and lemon juice until the spinach has wilted.
Season with salt and black pepper to taste before serving.