Wash and thinly slice the leeks, using only the white and light green parts. Chop Yukon gold potatoes into uniform chunks and place in the slow cooker.

Add diced butter, salt, dried thyme, and chili flakes to the slow cooker. Place a bay leaf on top and stir gently until mixed.

Pour enough chicken or vegetable broth to cover the vegetables completely and gently stir to incorporate all ingredients.

Cover the slow cooker and set to cook on low for 8 hours or high for 4 hours until the vegetables are tender.

Remove the bay leaf and check that the vegetables are tender. Puree the soup until smooth using a stick blender or standard blender.

Stir in heavy cream to the pureed soup, adjust consistency, and warm for an additional 10 to 15 minutes on low.

Ladle the soup into bowls and serve with crusty bread or fresh salad.