1 ripe pineapple--trimmed, cored and chopped (8 cups of pineapple chunks)
1 cup fresh orange juice
3-inch piece fresh ginger--scraped, minced and divided
2 jalapeños--halved, seeded, minced and divided
1 vanilla bean, halved lengthwise
2 tablespoons apple cider vinegar
2 tablespoons light Japanese soy sauce or tamari
Juice of ½ lime
1 cup extra-virgin olive oil
Salt and freshly ground black pepper
2 tablespoons cumin seeds
2 tablespoons black peppercorns
1 tablespoon granulated sugar
1½ teaspoons kosher salt
Four 4-ounce yellowfin-tuna fillets
1 tablespoon grapeseed oil
1. Make the vinaigrette: In a blender, purée the pineapple chunks and orange juice until completely smooth. Into a medium saucepan, pour the pineapple-orange mixture and place over medium-high heat. Add one-third of the ginger, half of the jalapeños and the vanilla bean and bring to a simmer. Reduce the heat to medium-low and cook until reduced to 2 cups, about 1 hour. Turn off the heat and set aside to steep for 15 minutes. Strain through a fine-mesh sieve into a medium bowl.
2. To the strained pineapple liquid, whisk in the cider vinegar, soy sauce, lime juice, the remaining ginger and the remaining jalapeño. While whisking slowly, drizzle in the olive oil. Season with salt and pepper; cover the bowl with plastic wrap and refrigerate to chill.
3. Make the fish: In a small skillet set over medium heat, toast the cumin seeds and peppercorns, shaking the pan often, until the cumin is fragrant and wisps of smoke rise from the pan, 30 seconds to 1 minute. Turn the spices out onto a plate to cool, then transfer to a spice grinder (or coffee mill) and pulverize until finely ground. Transfer to a small bowl and stir in the sugar and salt. Place the tuna steaks on a plate and sprinkle all sides with the spice rub.
4. Heat a grill pan or heavy-bottomed skillet over high heat for 2 minutes. In a small dish, add the grapeseed oil and dip a silicone or pastry brush (or folded paper towels and tongs) into the oil and use to grease the pan. Place the tuna steaks in the pan and sear, without moving, until browned, about 5 minutes. Use a metal spatula to turn the tuna steaks over and sear on the opposite side until they are browned, about 5 minutes more. Transfer the tuna to a cutting board, slice crosswise and divide among four plates. Drizzle with the pineapple vinaigrette and serve.