Preheat your oven to 350°F (175°C).

Lightly spray a large baking sheet with non-stick spray. Arrange the diced celeriac and onion in a single layer on the sheet. Roast in the oven for about 30 minutes, or until the vegetables are soft and lightly caramelized. The roasting process enhances the earthy flavors of the celeriac, giving the soup a deeper, more complex taste.

For the garlic: Drizzle olive oil into a small ramekin and place the garlic head cut-side down. Sprinkle with sea salt, cover with aluminum foil, and roast alongside the other vegetables. Roasting the garlic softens its natural sharpness and adds a sweetness to the soup.

Once the vegetables are tender, transfer them to a large bowl. Squeeze the roasted garlic out of its skin and add it to the bowl with the roasted vegetables.

Pour in enough vegetable stock to cover the vegetables. Use an immersion blender to purée the mixture until smooth and creamy. This is where the velouté technique shines, creating a silky texture without any cream.

For a truly smooth consistency, press the purée through a fine sieve, ensuring a velvety texture with no lumps.

In a small frying pan, melt the butter over medium heat. Add the focaccia pieces and toast until golden brown and crispy on all sides. These croutons provide a satisfying crunch, perfectly complementing the smoothness of the soup.

Reheat the velouté if necessary, and then ladle it into bowls.

Garnish with the crispy focaccia croutons. The toasts add not only a textural contrast but also a burst of flavor that elevates the soup.

Serve immediately, and enjoy the creamy richness of this delightful, light soup.