Preheat your oven to 450 F. Grease a baking pan or line with parchment.
Cut cauliflower into florets, then slice so one side of each floret is flat.
Arrange in a single layer in the greased pan. Bake 10 minutes on the center rack.
Meanwhile, whisk together the soy sauce, sweetener, vinegar, garlic, sesame oil, and ginger in a saucepan. Bring to a boil.
While waiting, stir together the cornstarch and water until cornstarch dissolves fully, then slowly whisk this into the saucepan as soon as it boils.
Turn heat to medium and cook 2 minutes, stirring more frequently once it returns to a boil. Cook until thick.
Flip cauliflower florets and bake 10 additional minutes.
If desired, you can now move the pan to the top rack and broil 1-2 minutes.
Pour sauce over florets. Sprinkle sesame seeds and optional scallions on top, and serve.
