In a heavy bottom stock pot heat up the olive oil and sauté the onion with a pinch of salt until translucent. Add the carrot, celery root and stalks and sauté everything together until the vegetables start to soften, about 10 minutes. Add the potato and bay leaves, cover with the water and bring to a gentle simmer. Cook on low flame until the potatoes are cooked through, about 10 minutes. Remove the soup from heat, discard the bay leaf, and add the arugula. Stir until wilted.
Using a hand held blender puree the soup until smooth. Adjust seasonings to your taste with the sea salt.
In a small skillet heat up the butter and add the panko bread crumbs. Stir and toast together on small flame until golden.
Ladle the celery soup into bowls and serve with a dollop of creme fresh, a drizzle of extra virgin olive oil and a good sprinkling of the crispy bread crumbs. Garnish with a few flakes of volcanic sea salt or any kind of coarse sea salt flakes.