Rinse barramundi and pat dry with a paper towel. Rub with lemon zest.
Sift flour onto waxed paper. Sprinkle garlic powder, salt, and pepper over the flour. Cover both sides of the barramundi with the flour mixture. Shake off excess flour.
Heat olive oil and canola oil in a skillet over medium heat. Place barramundi in the skillet, skin side-down. Cook until lightly golden in color, 3 to 4 minutes per side. Transfer to a plate using a broad spatula. Drizzle with truffle oil. Garnish with 1/4 of the lemon slices.