In a mortar, coarsely grind the chile with the coriander seeds. Add the garlic and mash to a paste. Stir in the basil, cilantro and lemon juice. Season the herb paste with salt and pepper.

Lightly brush a grill pan with vegetable oil and set it over moderately high heat until very hot but not smoking. Season the tuna steaks with salt and pepper and rub the herb paste all over both sides. Sear the tuna steaks for 1 minute per side for rare or until cooked to desired doneness. Serve at once.