Cure the Cod: Slice the cod loin into long, uniform rectangular batons (about 2cm thick). Season generously on all sides with salt. Wrap tightly in plastic wrap and refrigerate for 30 minutes to firm up the texture. Rinse the salt off under cold water and pat the fish bone-dry with paper towels.

Roll the Mosaic: Place beetroot powder in a small tray. Roll each cod baton in the powder until evenly coated on all sides. Lay three layers of plastic wrap on your workstation. Stack the coated cod batons together to form a cylinder. Roll the plastic wrap over the fish, pulling it as tight as possible to ensure the pieces fuse. Tie the ends securely. Optional: Vacuum seal the wrapped roll to remove any remaining air pockets.

Gentle Cook & Chill: Prepare a sous-vide bath at 50°C (122°F). Submerge the fish roll and cook for 30 minutes. Immediately move the roll to an ice bath for at least 20 minutes. Note: Slicing white fish while warm will cause the mosaic to shatter; it must be ice-cold.

Quick Pickle Carrots: Use a peeler to create long ribbons from the carrot. In a small saucepan, bring water, white wine vinegar, orange zest, star anise, and sugar to a boil. Place carrot ribbons in a heat-proof bowl and pour the boiling liquid over them. Let cool to room temperature.

Plating: Unwrap the chilled cod and use a very sharp, thin knife to slice the roll into 2cm thick 'medallions.' Place a mosaic slice in the center of each plate. Add a neat quenelle or dollop of crème fraîche next to the fish. Top the fish with a small spoonful of black caviar if using. Drape the pickled carrot ribbons artistically around the plate. Garnish with fennel fronds and a light drizzle of olive oil to add a professional sheen.