Rinse and drain the cooked lentils if needed and set aside.
Chop the carrot, celery, onion, and garlic finely.
In a medium pot, heat the olive oil over medium heat. Add the chopped onion and garlic, and sauté until fragrant and translucent.
Add the chopped carrot and celery to the pot, stirring occasionally until they start to soften, about 3-4 minutes.
Pour in the diced tomatoes along with their juices, and stir in the cooked lentils.
Bring the mixture to a gentle simmer and let cook for about 10 minutes so the flavors meld together.
Stir in the fresh spinach and cook until just wilted, about 1-2 minutes.
Season with salt, pepper, and any additional herbs you prefer. Adjust seasoning as necessary.
Serve the hearty lentil stew warm, enjoying it as a nourishing meal any time of the day.