Put the coriander, garlic, spices and lemon juice into a blender and blitz to a purée. With the motor running, slowly add the olive oil until you get a smooth, thick sauce. Set aside.
Sit the tuna steaks in a non-metallic dish and cover with two-thirds of the sauce. Cover with cling film, then leave to marinate in the fridge for about 20 mins (or for up to 4hrs).
Heat a griddle pan or grill. Shake off any excess marinade, season the tuna steaks, then cook for 2-4 mins, depending on thickness for medium rare, turning once (cook 2 mins more for well done). Drizzle over the remaining sauce paste to finish. Try serving with new potatoes; for a Moroccan twist, toss melted butter, harissa spice mix and chopped coriander leaves through the potatoes.