Combine lentils and water in a large pot. Bring to a boil. (Don't leave the pot unattended, as lentils have a tendency to be quiet one moment and boil over furiously the next.) Reduce heat, cover and simmer gently, stirring occasionally for 25 minutes or until the lentils dissolve completely and the soup looks relatively smooth.
Melt the butter in a small saucepan over medium heat. Stir in the coriander, chili powder, salt, cinnamon, cumin, garlic powder, cardamom and cayenne (if using), and cook 30 seconds or until fragrant. Stir in honey, then scrape the mixture into the lentils and stir well to combine.
Ladle into four bowls and garnish with croutons and pepitas.
Best Health tip: The soup lasts well in the fridge for up to three days. Reheat gently on the stove.
Using a slow cooker? Begin by reducing water to 6 cups. Combine lentils in a slow cooker and cook on low for eight hours or on high for three hours. The lentils should be completely dissolved and smooth after you stir them a few times. Cook the spiced butter on the stove and add to the soup.