Peel celeriac and potatoes and cut into medium size cubes. Heat olive oil in a large soup pot. Add onion and garlic and sauté until they begin to brown. Add potatoes and celeriac and stir everything together. Pour in enough vegetable broth or water to cover vegetables. Bring to a simmer and cook until potatoes and celeriac are very tender. Using an immersion blender, blend everything until smooth. If you don’t have one, you can also use a regular blender and purée in batches. Add more water if consistency is too thick. Stir in cream and season with thyme, sea salt and pepper, to taste. Slice onion batard roll and toast until slightly crispy; then tear into bite-size pieces. Divide soup between two bowls and top with crispy bread.