Have tuna sliced as thick as possible cutting against the grain into 1 1/2-inch rectangular pieces. Rub the fish with olive oil and roll in thyme, peppercorns, salt, and pepper. On a very hot grill sear the fish on each side for 30 seconds to 1 minute depending upon the desired doneness. Set aside and slice into 1-inch thick pieces.
For wasabi mayonnaise, start with 1 tablespoon of wasabi powder mixed with mayonnaise and add more, if desired, to taste.
Slice ginger into very thin sticks about 1-inch long. Place ginger, sugar, water and lemon juice into heavy saucepan and reduce to thick syrup. Be careful not to boil past medium ball state.
On individual washed white endive leaves place fish. Garnish with wasabi mayonnaise that has been put in a squeeze bottle and finish with a few pieces of ginger confit.