Season the thick block of tuna with salt and pepper and sear briefly on all sides in a hot sauté pan with olive oil, leaving the tuna very rare in the center. Set aside to cool. In a bowl mix the shaved fennel, orange segments, olives, olive oil, orange juice, and onion slices, season with salt and pepper, and arrange on a serving plate. Garnish plate with chopped parsley and sliced piquillo peppers. Once the tuna has cooled, slice it into ¼-inch slices and top the fennel salad with the slices, sprinkle a little sea salt on the tuna if desired, and serve immediately.