Heat oil in a saucepan over medium-high heat. Add cumin, coriander, and turmeric and cook stirring constantly for about 15 seconds. Be careful not to burn spices.
Stir in chili flakes, onion, celery, garlic, and ginger. Cook over medium heat for about 4 minutes, until vegetables are soft.
Stir in lentils and cook for 30 seconds, coating them in the oil and spices. Pour in water or stock, cover, increase heat to high and bring to a boil. Reduce heat and simmer for about 20 minutes until lentils are tender. Stir in lemon juice and season with salt and pepper to taste.