Finely dice the tuna and slice the spring onion very thin.
Combine in a bowl with salt, lime zest and juice.
Let marinate at least 1 hour in the fridge before serving.
Serve in individual glasses or on a plate pressing well in a food ring mold.
Decorate with edible flowers or edible sprouts.
Notes
If you're not sure of how the tuna has been handled/processed, to be safe you should freeze it for at least 15 hours, according to the FDA.Thaw it in the fridge before consuming.