Wipe both sides of the tuna steaks with damp paper towel. Mix the olive oil, lemon juice and seasoning together and drizzle over the fish, turning each steak to ensure that both sides are coated with the marinade. Set aside while you make the pesto.
To make the pesto, dry-roast the almonds over a medium heat for 5 minutes or until they darken. Set aside to cool then mix with the other pesto ingredients in a food processor until well combined but still quite grainy. Refrigerate until ready to serve.
Heat a well-seasoned chargrill pan or barbeque and cook the tuna steaks for 3-5 minutes each side, turning them once.
Serve on a bed of spinach and top with the pesto sauce.