Ingredients

For the rosemary oil:
1 cup olive oil
4 (4-inch) sprigs fresh rosemary

For the soup:
2 tablespoons olive oil
1 medium yellow onion, chopped (about 1 cup)
2 garlic cloves, minced (about 1 tablespoon)
1 quart low-sodium vegetable stock, plus more as needed for reheating
1 medium head cauliflower, cored and broken into 1 1/2-inch florets (about 2 1/2 pounds)
2 teaspoons kosher salt, plus more to taste
1 teaspoon black pepper, plus more to taste
Freshly grated zest of 1 lemon, for serving

For the croutons:
3 cups diced rustic country bread (3/4-inch pieces)

Preparation

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 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, 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.

Meanwhile, make the optional croutons: Heat the reserved skillet with the residual rosemary oil over medium. Add the bread cubes, sprinkle generously with salt and pepper, and cook, tossing often, until toasted all over, 4 to 5 minutes. Transfer the croutons to a plate or board to cool.

Strain and discard the rosemary stems from the rosemary oil. Working in batches if necessary, carefully transfer the vegetables, stock and 1/4 cup rosemary oil to a blender and blend on high until creamy. Add more rosemary oil to taste, and blend to combine. Return the soup to the pot 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.

Serve hot. Garnish each serving with a swirl of rosemary oil, a few croutons, and a sprinkle of lemon zest. The soup will thicken as it sits; add more stock as necessary when reheating. Leftover rosemary oil will keep in a sealed container at room temperature for up to 1 week.