Preheat oven to 425°F.
Toss chopped celery root, rutabaga, parsnip and apple in a large bowl with olive oil, thyme, and a pinch of salt.
Spread onto 2 oiled or lined rimmed baking sheets.
Roast until soft and golden brown (especially rutabaga), about 45 minutes, switching position of sheets halfway through cooking.
Combine roasted veggies and 11/2 cups stock in a blender or food processor, in batches if necessary (or use an immersion blender).
Blend until smooth.
If too thick, add more stock until desired consistency is reached.
Garnish with croutons if desired.