Make the rosemary oil. In a small, heavy saucepan, combine the rosemary sprigs and olive oil. Bring to a very gentle simmer over medium low heat. Cook for about 5 minutes, adjusting heat as needed. Be careful not to fry the rosemary. Remove from heat and allow the rosemary to cool in the oil while you make the soup.

Make the soup. In a large, heavy pot or Dutch oven, heat the olive oil (not the rosemary oil). Add the onion and cook, stirring occasionally, until tender and translucent, 6 to 8 minutes. Add the garlic and cook until fragrant, about 30 seconds.

Add the stock, cauliflower, salt (start with 1 teaspoon if the stock you are using is not low sodium) and pepper and bring to a boil over high heat. Lower the heat, cover, and simmer until the cauliflower is tender and can be pierced easily with a fork, about 10 minutes.

Meanwhile, make the croutons. Lightly coat a skillet with some of the reserved rosemary oil and heat over medium. Add cubed bread and sprinkle with salt. Cook, tossing often until golden and toasted all over, about 4 to 5 minutes. Transfer the croutons to a plate to cool.

Strain the rosemary infused olive oil into a glass jar or bowl and discard the rosemary. Transfer the stock and vegetables to a blender, working in batches if necessary, and add 1/4 cup rosemary oil. Blend on high until the soup is completely smooth and creamy. Add more rosemary oil, salt and pepper to taste and blend to combine. Return the soup to the pot and keep warm. If the soup seems thin, simmer another 5 to 10 minutes to reduce slightly. The soup will continue to thicken as it cools.

Serve hot, garnished with a drizzle of rosemary oil and a few croutons. If the soup seems too thick when reheating, add a little stock. The soup will keep refrigerated for a couple of days. The rosemary oil will keep in a sealed container at room temperature for about a week.