For the marinade, zest and juice the limes and combine with the olive oil and cardamom. Reserve a little for serving.
Marinate the tuna in the marinade for 24 hours.
Peel the echalions, cut them in half. Cook the echalions on the barbecue for 15 minutes. Drizzle the echalions with the reserved marinade, season with sea salt. Serve immediately with tuna.
Sear the tuna fil