Preheat a grill pan to medium-high. In a bowl, whisk the oil, garlic, Chinese five-spice powder, 1/2 teaspoon salt and 1/4 teaspoon pepper. Add the shrimp and toss to coat. In a bowl, toss the scallion and sesame seeds.
Grill the shrimp, turning once, until just firm and opaque, about 5 minutes. Sprinkle the scallion-sesame seed mixture on top.