Fresh Tuna Salad with Avocado

Name: Fresh Tuna Salad with Avocado
Ingredients: 1/2 cup extra-virgin olive oil; 1 tablespoon fennel seeds; 1 tablespoon black peppercorns; One 3/4-pound tuna steak, cut 1 inch thick; Salt; 1/2 cup plus 2 tablespoons mayonnaise; 2 anchovy fillets, minced; 2 tablespoons capers, drained and chopped; 1 garlic clove, minced; 2 tablespoons fresh lemon juice; 2 celery ribs, sliced crosswise 1/4 inch thick; 1 small red onion, finely diced; 1/3 cup pitted oil-cured black olives, chopped; 4 Hass avocados, halved and pitted; Pea shoots or alfalfa sprouts, for garnish (optional)