Place a large pot over medium heat and add the bacon. Cook until crispy, then remove from the pot and place on a paper towel-lined plate to drain off excess grease.
In the same pot, add the butter and allow it to melt over medium-high heat. Add in the leeks and cook for approximately 5 minutes, or until they are tender.
Next, add the potatoes and chicken broth. Bring the mixture to a boil, then reduce the heat and let it simmer for approximately 15 minutes.
Once the potatoes are cooked through, use an immersion blender or transfer the soup to a blender and blend until smooth. Return the soup to the pot if using a blender.
Finally, add in the thyme, onion, and grated cheese. Allow the soup to simmer for another 10 minutes until the cheese is completely melted and the onions are cooked through. To finish, stir in the heavy cream and add salt to taste. Serve hot with crumbled bacon on top for an added touch of indulgence.