Place the tofu in a shallow sauce pan and cover with cold water. Optionally cut the block of tofu in half.
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, Korean chili 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 or bowls.
Spoon the sauce over the tofu and serve warm.
