Season tuna with kosher salt and fresh cracked pepper.
Place arugula and capers on a plate. Combine oil and lemon juice, salt and pepper.
Heat your grill to high heat and clean grate well. When grill is hot, spray grate with oil to prevent sticking then place tuna on the grill; cook one minute without moving. Turn over and cook an additional minute; remove from heat and set aside on a plate.
Slice tuna on the diagonal and place on top of salad. Top with lemon vinaigrette and eat immediately.