Cover the tofu slices with paper towels on a cutting board. Place another cutting board on top and weight it with a heavy pan or a couple of cans. Let it drain on an angle for about 2 hours to remove any excess moisture.
In a shallow container large enough to hold the tofu in a single layer, whisk together the soy sauce, lemon juice, sugar and sesame oil. Add the pressed tofu, turning to coat, then cover and refrigerate for 3 hours.
Preheat a chargrill pan or barbecue hotplate to medium. Remove the tofu from the marinade, reserving the marinade. Place the tofu on the grill tray and cook for 5 minutes on each side, or until golden.
To serve, cut each piece of tofu into four triangles and sprinkle with the sesame seeds, spring onions and any leftover marinade.