Heat the sesame oil in a frying pan over medium-high heat.
When the oil is hot, sear the tuna 30 seconds on each side. When all sides are seared, remove from heat and let it cool.
Slice the tuna into 1/4-inch pieces.
Pour the ponzu sauce and sweet chili sauce over the fish. Garnish with green onion, sesame seeds, and sliced lemon.