Arrange tuna steaks in one layer in a shallow dish and lightly brush both sides with oil.
Spread peppercorns and dill evenly on both sides of tuna and marinate 20 minutes.
Grill tuna on a oiled rack set about 5-6 inches over glowing coals. 3 minutes on each side for medium-rare fish. (Alternatively, grill tuna in a preheated oiled well-seasoned ridged grill pan).
Make sauce while tuna marinates:
In a heavy saucepan combine scallions, coriander, lime juice, soy sauce and wasabi paste and bring to a boil.
Reduce heat to low and whisk in butter, 1 piece at a time, lifting pan from heat occasionally to cool mixture and adding each new piece before previous one has melted completely. (Do not allow sauce to simmer. Sauce should not get hot enough to seperate).
This dish goes well served with garlic mashed potatoes and french green beans.