In a large saucepan over medium-high heat, melt butter and sauté onion and garlic with rutabaga and celeriac. Add the chicken broth and thyme. Season generously and bring to boil.
Simmer over low heat for 20 minutes.
Meanwhile, in a blender, blend apple and apple juice. Heat and reserve for serving. Remove thyme sprigs and purée soup with a handheld blender or regular blender. Add milk while blending.
Ladle soup into bowls and garnish with a drizzle of apple purée, diced Oka and a few leaves of thyme.