Rinse the lentils under cold water, removing any debris or stones, and set aside to drain.
In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until translucent. Add minced garlic and cook for an additional minute until fragrant.
Stir in the diced carrots, celery, and bell pepper. Sauté for another 5-7 minutes until the vegetables start to soften.
Sprinkle the cumin and coriander over the vegetable mixture. Stir well to coat and cook for about 1-2 minutes to enhance flavors.
Add the rinsed lentils, vegetable broth, and diced tomatoes to the pot. Bring to a boil, then reduce heat to low.
Cover and simmer for about 30-35 minutes, or until lentils are tender. Stir occasionally.
Once cooked, taste and season with salt and pepper as needed. Serve hot, garnished with fresh parsley.