Season tuna on both sides with salt. Dust 1 side of the tuna with the peppers. Heat oil in a large saute pan over medium-high heat. Sear the tuna, pepper side down, until golden brown, about 1 to 2 minutes, turn over and continue cooking for 2 minutes for medium-rare doneness.
To plate, divide the melted leeks onto 4 serving plates. Arrange each tuna steak on top of the melted leeks. Place a tablespoon of the Tangerine-Green Onion Relish on top of the tuna, and drizzle the entire dish with the Red-Wine Pomegranate Reduction.
Tangerine-Green Onion Relish:
Combine all ingredients in a medium bowl and season with salt and pepper to taste.
Place leeks, onion, carrot, celery, and water to cover in a large pot and season with salt and pepper. Bring to a simmer and cook until cooked through, about 20 to 25 minutes. Drain well and thinly slice.
Heat oil and butter in a large saute pan over medium heat. Add leaks, season with salt and pepper and cook for 10 to 15 minutes.
Red Wine-Pomegranate Reduction:
Place wine and port in a small saucepan and reduce to 1 cup. Whisk in pomegranate molasses and sugar and taste for sweetness. Whisk in cold butter just before serving and season with salt and pepper, to taste.