Saute leeks and onion in olive oil for 10 min on med/low heat until soft and fragrant.
In a large pot, boil broth. Add sautéed leeks and onion, garlic, potato cubes and spices. Cover and simmer for 20-30 min, until potatoes are very soft and fall apart when forked.
Puree the soup with a blender or immersion blender. Make sure not to fill the blender more than 1/2 way so soup doesn't go flying (blend portions at a time). Pour soup back in pot and reheat. Season to taste with salt and pepper.