Place the tofu in a shallow sauce pan and cover with cold water. Cut the block of tofu in half if desired.
Bring to a simmer over medium-high heat, then keep warm, covered, over very low heat for at least 20 minutes.
Meanwhile, mince and mash the garlic to a paste with a pinch of salt.
Stir together the garlic paste with the remaining ingredients (scallion, sesame seeds, soy sauce, sesame oil, red-pepper flakes, and sugar).
Just before serving, carefully lift the tofu from the saucepan with a large spatula and drain on a towel.
Transfer the tofu block(s) to a shallow bowl.
Spoon the sauce over the tofu and serve warm.
