Preheat oven to 350°F. Line a large baking sheet with parchment paper. Spread panko crumbs in a thin even layer across the surface of the baking sheet. Spray panko crumbs with cooking oil spray. Bake crumbs about 5-7 minutes or until golden brown.
Prepare cauliflower: Set whisked eggs aside in a small bowl. Line another large baking sheet with parchment paper.
Dip cauliflower in egg mixture and then shake a few times so that excess egg drips off. Then place into sheet with panko crumbs. Cover cauliflower with panko, pressing the crumbs gently against the cauliflower to help them stick. Place coated cauliflower on empty prepared baking sheet. Repeat until all cauliflower is coated.
Bake for about 15-20 minutes or until cauliflower is cooked and the coating is a dark golden brown.
Make sauce: While the cauliflower is cooking, dissolve cornstarch in water and set aside. Add all sauce ingredients except the cornstarch water to a small pot or saucepan. Bring to a gentle simmer and stir a few times until ingredients are mixed. Then add cornstarch water to the sauce. Stir occasionally and let sauce come to a simmer and cook until sauce starts to thicken (about 2 minutes).
Serve: Allow sauce to cool a few minutes and thicken even more. Drizzle over cauliflower and garnish with scallions.