In a large heavy saucepan, heat the olive oil over medium heat. Add the chopped bacon and cook until crispy. Use a slotted spoon to transfer the bacon to a bowl, leaving the fat in the pan.

Add the shiitake mushrooms to the pan and cook over medium-high heat until golden. Transfer to the bowl with the bacon.

Add the onion, celery, and carrots to the pan. Cook over medium heat until softened, about 5-7 minutes. Add the minced garlic, thyme leaves, and bay leaves. Cook for another 5 minutes.

Stir in the diced sweet potato, lentils, and broth. Bring to a boil, then reduce to a simmer. Cook for 15 minutes, stirring occasionally, until the lentils are tender.

Remove 1 cup of the soup and blend until smooth. If the mixture is too thick, add ¾ cup of boiling water. Return the blended mixture to the pot and stir to combine.

Season the soup to taste with salt and pepper. For extra flavor, add a splash of sherry or balsamic vinegar. To serve, ladle the soup into bowls, top with the reserved bacon and mushrooms, and drizzle with single cream.