Prepare Ingredients: Rinse the red lentils thoroughly and pick out any debris. Chop the onions, garlic, carrots, and celery. Measure out the vegetable broth and spices.
Sauté Aromatics: In a large pot over medium heat, sauté the chopped onions and garlic until fragrant, about 2-3 minutes.
Add Vegetables & Lentils: Stir in the carrots, celery, and red lentils. Mix well with the sautéed onions and garlic.
Simmer the Soup: Pour in the vegetable broth, diced tomatoes, cumin, coriander, and thyme. Bring the mixture to a boil, then reduce the heat and let it simmer for about 20 minutes, until the lentils are tender.
Blend for Creaminess (Optional): Use an immersion blender to blend the soup until smooth, or leave it chunky for a more rustic texture.
Season & Finish: Stir in lemon juice (if using) and season with salt and pepper to taste.
Serve & Garnish: Ladle into bowls, garnish with fresh herbs, and enjoy with a side of crusty bread or a fresh salad.