Preheat the oven to 400°F (200°C) and line a baking sheet with a silicone mat or parchment paper.

Spread the cauliflower florets and chopped fennel on the baking sheet, drizzle with the olive oil, and season with salt and pepper. Toss to coat, then roast for 20 to 25 minutes, turning once, until the vegetables are tender and browned at the edges.

Let the vegetables cool for 5 minutes, then transfer them to a blender. Add the broth and blend until completely smooth, venting the lid so the steam can escape.

Pour the soup into a saucepan over medium-low heat. Stir in the heavy cream, season to taste with salt and pepper, and heat for about 5 minutes until steaming hot (don't let it boil). Ladle into bowls and garnish with fennel fronds, if using.

To make the crostini (optional), heat the broiler. Mix the softened butter, garlic paste, and Italian seasoning in a small bowl. Spread it over the bagel chips, arrange them on a baking sheet, and broil for 1 to 2 minutes until the butter melts and sizzles. Watch closely so they don't burn.