Slice each block of tofu in half horizontally. Lay the pieces of tofu on a cutting board and place a can or small bowl under one end of the board to tilt it slightly. Set the board so that the low end hangs over the sink. Cover the tofu with paper towels, place another cutting board on top and weight it with a heavy pan or a couple of cans. Let drain for 2 hours.
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. Place the pressed tofu in the pan and let stand for 3 hours or until the marinade has been absorbed about halfway up the tofu (there should still be some marinade in the container).
Preheat the grill to moderate. Remove the tofu from the marinade, reserving any left over. Place the tofu on the grill tray and cook for 5 minutes on each side or until richly browned.
Meanwhile, in a small frying pan, toast the sesame seeds over low heat for 3 minutes or until golden.
To serve, cut each piece of tofu into four triangles and sprinkle with the spring onions, sesame seeds and any leftover marinade.
Some more ideas…
If you can find pressed tofu, skip step 1. Pressed tofu has already had much of its moisture pressed out of it.