1 Crush black and white peppercorns in a mortar and put on a plate.
2 Cut tuna lengthwise into 2 cylindrical pieces.
3 Roll tuna pieces in peppercorns to coat.
4 Cut leek diagonally into 1 cm (approximately 1/2-inch) slices.
5 Carefully rinse leek slices in several changes of cold water, keeping rounds intact if possible. Remove and drain well.
6 Peel the garlic and thinly slice. Coarsely chop the olives.
7 Rinse the parsley, shake dry, pluck leaves and coarsely chop. Squeeze orange to yield 150 ml (approximately 2/3 cup) juice.
8 Heat 1 tablespoon oil in a pan over medium heat. Cook leek and garlic, stirring frequently, until softened, 3 to 4 minutes.
9 Add olives and orange juice and bring to a boil. Cover and cook until slightly reduced, about 3 minutes, then season with salt and pepper.
10 Meanwhile, heat the remaining oil in a skillet. Cook the tuna loins until seared and golden, about 45 seconds per side.
11 Combine parsley and leek mixture and arrange on a plate. Cut tuna into slices, arrange on top, and serve.