Arrange cauliflower head stem down on a cutting board (trim stem if necessary so bottom of cauliflower can rest on cutting board). Using a sharp knife, cut cauliflower into slices about ¼" thick. (The ends will crumble and that’s okay; reserve pieces for another use. There should be 3 or 4 center 'steaks' that hold together.) Arrange cauliflower steaks in a single layer in a 13" x 9" baking dish.

In a medium bowl, combine soy sauce, garlic, vinegar, ginger, sesame oil, and ¼ cup cold water. Pour over cauliflower and turn to coat. Let sit, turning halfway through, 30 minutes.

Meanwhile, in a small pot, whisk cornstarch and 1 tablespoon cold water until cornstarch is dissolved. Add brown sugar and whisk to combine.

Carefully transfer cauliflower to a metal baking sheet; season with salt.

Pour marinade into pot with brown sugar mixture. Bring to a boil over medium-high heat and cook, stirring occasionally, until reduced and thick enough to coat the back of a spoon, 4 to 6 minutes. Remove sauce from heat.

Meanwhile, prepare a grill for medium heat (or heat a grill pan over medium heat); preheat 5 minutes. Clean and oil grates with neutral oil.

Arrange cauliflower steaks on grill, spacing evenly apart. Grill, undisturbed, until deeply marked on the bottom, 6 to 8 minutes. Using a large metal spatula, carefully flip steaks. Continue to grill until grill marks appear on the bottom and steak is tender enough to pierce with a fork, 6 to 8 minutes more.

Arrange cauliflower on a platter. Slather with teriyaki sauce. Top with scallions and sesame seeds.