To begin, gather all your ingredients so that you have everything close at hand on the counter or table.
Prepare the Caper Sauce
Using a knife and cutting board, chop the parsley. Mince the garlic.Zest the lemon to make 1 teaspoon. Squeeze the lemon to make 2 tablespoons fresh lemon juice.
Place the capers, minced garlic, lemon zest, and red pepper flakes inside a mortar bowl. Mash the ingredients with pestle to make a loose paste.
Transfer the capers to a small bowl, and add chopped parsley and lemon juice. Stir in the kosher salt, and freshly ground black pepper.
Whisk in the extra virgin olive oil to finish the sauce. Set aside.
Cook the Tuna
Pat the tuna steaks with paper towels to remove excess moisture. Brush each with olive oil on both sides.
Sprinkle with kosher salt, and freshly ground black pepper.
Heat a large heavy skillet (a cast iron grill pan works great) on medium-high heat. Brush the grill with olive oil.
Add tuna steaks to the pan and sear on each side for 3-4 minutes, brushing with more oil before turning. If desired, turn steaks 90 degrees to create cross-hatch marks. Adjust the heat as needed, if the skillet gets too hot. Steaks should be opaque and slightly pink in the center.
To serve, place tuna steaks on individual plates and spoon sauce over the steaks. Garnish with fresh parsley is desired.