Saute onions, carrots, potatoes, & celery in large pot, Cook until veggies are soft and aromatic.
Add water, chicken (or veg) stock, bay leaf, and lentils to veggies. Add vegetable soup if using. Bring to a boil and simmer for at least 5 minutes, stirring occasionally. Add 1 tsp cumin & rest of spices, stir in and taste, add more. I find the cumin flavor kind of disappears with long cooking so I usually add half in the beginning and half about 15-30 minutes before serving.
Let cook on low for as long as you have time. Flavor of soup improves with longer cooking.
Blend soup in blender or with immersion blender until texture is mostly smooth.
Serve with freshly ground black pepper, croutons and wedges of lemon to squeeze over the soup (essential!).