1. Holding your knife parallel to the cutting board, cut each block of tofu in half through the side. Cut each of these pieces in half again to form a total of 8 broad, thin slabs. Blot the tofu pieces dry with paper towels, then place them in a nonreactive baking dish.
2. Combine the port, vinegar, orange juice, soy sauce, honey, and 4 tablespoons of the sesame oil in a nonreactive bowl and whisk to mix. Pour this mixture over the tofu and let marinate in the refrigerator, covered, for at least 2 hours or as long as overnight, turning the tofu pieces several times so they marinate evenly.
3. Stem the spinach, rinse it, and spin it dry in a salad spinner or shake it dry. Place the spinach in a large nonreactive bowl.
4. Set up the grill for direct grilling (see page 21 for charcoal or gas) and preheat to high.
5. Drain the marinade from the tofu into the spinach and toss to coat the spinach.
6. When ready to cook, brush and oil the grill grate. Lightly brush the tofu pieces on both sides with sesame oil. Arrange the marinated tofu pieces on the hot grate, placing them on a diagonal to the bars. Grill until lightly browned on both sides, 3 to 4 minutes per side. Transfer the grilled tofu to a platter or plates.
7. Place the spinach on the grate and cook until wilted (the edges will crisp), 2 to 4 minutes per side, turning the leaves with tongs. Place the grilled spinach on top of the tofu. Top with the orange segments, sprinkle the toasted celery seed over all, and serve at once.