Make the rosemary oil: In a medium skillet, combine the olive oil and rosemary sprigs. Cook over low heat for 5 minutes, lowering the heat if the oil reaches a full simmer. (You want to cook it at a very gentle simmer to avoid frying the rosemary.) Carefully pour the oil and rosemary into a small bowl, leaving a slick of oil in the pan if you plan to make croutons. Allow the rosemary to cool completely in the oil while you make the soup.

Make the soup: In a heavy pot or Dutch oven, heat 2 tablespoons of the rosemary olive oil over medium-low. 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. (Be careful not to let the garlic scorch!)

Add the stock, cauliflower, chickpeas, salt and pepper, and bring to a boil over high. Lower the heat, cover, and simmer until the cauliflower is tender when pierced with a fork, about 10 minutes.

Strain and discard the rosemary stems from the rosemary oil. Add ¼ cup rosemary oil to soup. With an immersion blender purée soup. Add more rosemary oil to taste, if necessary. Return the soup to heat and bring to a simmer. If the soup seems thin, let it simmer for 5 to 10 minutes to reduce slightly. (Remember: The soup will continue to thicken as it cools.) Season to taste with salt and pepper.