Melt the butter in a medium stockpot or Dutch oven over medium-high heat. Add the leeks and saute until softened and tender, 2 to 3 minutes, stirring occasionally. Add the flour and cook, stirring constantly, for about 1 minute. Slowly add the chicken stock, whisking until it is fully incorporated. Bring the mixture to a boil, and then lower the heat to allow the mixture to cook at a simmer for 20 minutes.

Cut the top 1/3 off the sourdough rounds and reserve for croutons. Hollow out the remaining bread round, leaving about 1-inch around the sides, and 1 1/2 to 2 inches on the bottom. Save 3 cups of the inside portion of the bread, chopped, for the soup. Set aside.

Cut the reserved top 1/3 portion of the bread rounds into a small dice, enough to equal 2 cups chopped. Place the bacon in a medium skillet over medium-low heat and cook until the bacon is browned and crisp and the fat is rendered. Remove the bacon with a slotted spoon and drain on paper towels. Set aside for a garnish. Increase the heat to medium and add the 2 cups of diced bread. Cook, stirring, until the bread is golden and toasted, about 7 minutes. Remove from the heat and set aside to use as a garnish with the soup.

Remove the simmered soup from the heat and add the reserved 3 cups of the sourdough removed from the inside of the bread rounds to the hot soup. Let the bread sit in the soup for 5 minutes. Add the cream and 6 ounces of the crumbled Stilton to the soup and, in batches, transfer the soup mixture to a blender or food processor. Process the soup until smooth and slightly thickened, about 30 seconds per batch. Return the soup mixture to the stockpot or Dutch oven and heat over medium heat until heated through. Season with salt and white pepper.

Place the hollowed out bread rounds onto plates and ladle the soup into the hollowed out portion. Garnish the soup with the remaining 2 ounces of crumbled blue cheese, the toasted sourdough croutons, and the bacon. Serve immediately.