Heat 45 ml of Oil in a saucepan over medium heat.
Add half of the onions and saute until softened, 3 - 4 minutes.
Add the cumin, coriander, red pepper flakes and garlic and mix well. Cook for 2 minutes.
Add lentils, carrot and celery leaves, reserving a few for garnish.
Pour in the vegetable stock and simmer until the lentils have almost broken down, 30 - 40 minutes.
Add the lemon juice and season to taste with salt and pepper.
Heat the remaining 30 ml of oil in a small frypan over medium heat and add the remaining onion, sauteing until crisp and golden, about 5 minutes.
Ladle the soup into bowls and garnish with the onions and reserved celery leaves.