3 tablespoons salted butter
In a large pot, melt the butter over medium-low heat. Add the celery root, season with salt and pepper and cook for 5 minutes. Pour in the milk and stock and bring the soup to a simmer. Cover, reduce the heat, and simmer until the celery root is tender, about 20 minutes.
Purée the soup, preferable using an immersion blender. Stir in the mustard and season with salt and pepper as needed. Serve hot, drizzling each portion with a little olive oil and sprinkling with a pinch of piment d'Espelette. Top each bowl with a slice of bacon.