Combine the hoisin sauce, ketchup, rice vinegar, soy sauce, sesame oil, ginger, and scallion in a small saucepan. Simmer over medium-high heat until the mixture is thickened and reduced to roughly 3/4 cup, about 5 to 7 minutes.

Prepare the grill for medium-high heat. Clean the grates thoroughly and wipe them with a small amount of vegetable oil using a folded paper towel—the oil should shimmer but not smoke.

Pat the tofu planks dry once more to ensure a good sear. Place the tofu on the grill and cook until distinct grill marks appear and the edges start to crisp, about 3 to 5 minutes per side.

Brush the top of each tofu plank generously with the remaining glaze. Flip the tofu and cook for 1 minute, then glaze the second side and flip again until the sauce is bubbling and slightly charred.

Serve immediately while hot, with the reserved glaze on the side.