Heat oil in pan over medium-high heat, add onion, garlic, celery, carrots and stir until softened. Stir in cumin and coriander and add lentils and tomato paste. Mix in stock, water, tomatoes, bay leaf and season with salt and pepper.
Bring to a boil, then reduce to medium-low heat and cover slightly with lid, cooking 15-20 minutes until lentils are soft. Remove bay leaf and discard.
With an immersion blender or regular blender, puree soup until smooth.
Serve with a dollop of plain Greek yogurt and your favourite whole-grain bread to dip!
