Preheat oven to 425 degrees. Toss onion, cauliflower and garlic with olive oil and a sheet pan. Season with salt and pepper. Roast vegetables until tender and caramelized, stirring occasionally, about 30-35 minutes.
Transfer roasted vegetables to a dutch oven or large saucepan. Add broth, water, rice and bay leaf. Bring to a boil and reduce to a simmer. Continue to cook until rice is tender, about 20 minutes.
While the soup simmers, make the Croutons. Toss bread cubes with olive oil on a rimmed baking sheet. Season with salt and pepper. Bake until toasty golden brown, about 7-9 minutes. Remove from the oven and set aside.
Remove bayleaf from the soup. Puree soup in two batches in a blender until smooth (or use an immersion blender if you have one). Return soup to pot.
Bring soup back to a simmer over low heat. Add lemon juice and season to taste with salt and pepper. Serve with Croutons, a drizzle of olive oil and a shower of chopped fresh chives.