Preheat the oven to 200 degrees Celsius / 392 Fahrenheit with top and bottom heat.
Cut off the top of the garlic bulb. Drizzle with olive oil and season with salt and pepper. Wrap it in aluminum foil and place it on a baking sheet.
Peel the celeriac. Cut the celeriac and cauliflower into large pieces. Mix with pepper, salt, and olive oil. Spread them out on the baking sheet next to the garlic.
Grill the vegetables in the oven for 30 minutes.
Heat a little olive oil in a large soup pot and add the grilled vegetables and three cloves of roasted garlic.
Add the broth to the vegetables and let the soup simmer for another 10 minutes.
Puree the soup until smooth using an immersion blender, then season to taste with salt and pepper. Garnish with parsley pesto and croutons, if desired.