Parsley or mint works if you don’t have any basil.
Ingredients
4 tuna steaks (3/4 to 1 inch think; 6 to 8 ounces each)
1 bunch fresh basil, washed and stemmed
4 cloves of garlic, cut in half
3 strips of lemon zest
Juice of 1 lemon (3 to 4 tbsp)
1 tbsp white wine vinegar
1 cup extra-virgin olive oil
1 tsp coarse salt
1 tsp black pepper
Directions
Combine all the ingredients in a food processor or blender and puree until smooth. Pour this mixture over the tune and marinate in the refrigerator, covered, for 30 minutes to 4 hours, turning the tuna steaks a few times. Save some to decorate the plate
Source: How to Grill: The Complete Illustrated Book of Barbecue Techniques, by Steven Raichlen,