How to Make It
Preheat oven to 350°F. Heat 2 tablespoons of the oil in a Dutch oven over medium. Add onion, and cook, stirring occasionally, 5 minutes. Add garlic, and cook, stirring occasionally, until onions are very soft and beginning to caramelize, about 3 minutes. Stir in broth and 3/4 teaspoon of the salt; increase heat to high, and bring to a boil.
Add cauliflower, reduce heat to medium, and simmer, uncovered, until cauliflower is tender, about 20 minutes.
Meanwhile, toss bread cubes with 1/4 teaspoon of the pepper, and remaining 1 tablespoon oil and 1/4 teaspoon salt. Arrange in a single layer on a rimmed baking sheet, and bake in preheated oven until browned and crisp, about 10 minutes, stirring halfway through.
Working in batches, if needed, pour soup into a blender; add butter. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening. Process until smooth, about 30 seconds. Divide soup among bowls, and top with croutons, chives, and remaining 1/4 teaspoon pepper.