For the croutons: Position a rack in the center of the oven and preheat the oven to 400 degrees F. Arrange the bread cubes in a single layer on a rimmed baking sheet. Drizzle with the olive oil. Sprinkle the thyme on top and season with the salt and pepper if using. Toss until coated. Bake until golden and crisp, 10 to 12 minutes. Set aside to cool.
For the bacon: On another baking sheet, arrange the bacon in a single layer. Bake until browned and crisp, 15 to 17 minutes. Drain on paper towels. When cool enough to handle, chop into 1/2-inch pieces and set aside.
For the soup: In a large saucepan or Dutch oven, melt the butter over medium-high heat. Add the celery, shallots, 1 teaspoon salt and 1/4 teaspoon pepper, and cook until soft, about 4 minutes. Add the garlic and cook until aromatic, about 30 seconds. Stir in the chicken broth, thyme and cauliflower. Bring the mixture to a boil. Reduce the heat to a simmer, cover and cook until the vegetables are tender, 20 to 25 minutes.
In batches, puree the soup in a food processor until smooth. Season with salt and pepper. Cool the soup to room temperature, then refrigerate until ready to serve, at least 2 hours.
Ladle the chilled soup into bowls and garnish with the croutons and bacon.
Cook's Note: The soup can also be blended using an immersion blender.