Preheat oven to 400°F. Line a large baking sheet with parchment paper. Place cauliflower florets on a baking sheet. Roast for about 15-20 minutes or until cauliflower is cooked and tender but still a little crisp. Allow cauliflower to cool for a few minutes (about 10 minutes).
While the cauliflower is cooking, make sauce on the stove. Add all sauce ingredients except the cornstarch + water. Bring to a low boil and stir until blended. In a small bowl, completely dissolve cornstarch in water, then add to the mixture. Stir until sauce boils again and cook until sauce thickens (about 2 minutes). Set sauce aside and allow to cool for a few minutes (it should still be quite warm, but not so hot as to burn your skin if you were to touch it). Sauce will thicken even more when cooling down.
When cauliflower has cooled down enough that you can touch them without burning yourself, add them to a large bowl. Pour in sauce and toss cauliflower in sauce and coat. Please allow cauliflower to cool down slightly before tossing in sauce, otherwise if they are too hot, it will thin out your sauce considerably. Serve while still warm. Garnish with fresh scallions if desired.
