In a medium saucepan, heat the oil over medium heat until shimmering. Sweat the leek, celery, carrot and yam until translucent, about 5 minutes. Add garlic, lentils, spices, bay leaves and salt and pepper. Cook, stirring constantly, until fragrant, 1-2 minutes. Add vegetable stock, increase heat to medium high, and bring to a boil. Reduce heat and simmer, uncovered, for 20-25 minutes, or until vegetables are soft and the lentils are breaking apart.
Remove from heat, remove bay leaves and use an immersion blender or blender to puree until smooth. Season to taste with salt and pepper.
Garnish with coconut milk, chopped apples and toasted walnuts.