Prepare the Cauliflower: Cut the cauliflower into bite-sized florets. In a bowl, mix the soy sauce, ketchup, oil, salt, garlic powder, and freshly ground black pepper to make the marinade. Toss the cauliflower florets in the marinade until they are well coated. Sprinkle the cornstarch over the cauliflower and toss again to evenly coat.

Roast the Cauliflower: Preheat your oven to 400°F (200°C). Spread the cauliflower in a single layer on a lined baking sheet and roast for 20-25 minutes, or until the florets are crispy and golden, flipping halfway through for even cooking.

Prepare the Sauce: While the cauliflower is roasting, heat 1 teaspoon of oil in a large skillet or wok over medium heat. Incorporate the chopped garlic, grated ginger, and dried red chilies, then stir-fry for 1 to 2 minutes until they release their aromatic flavors.

Cook the Vegetables: Add the chopped red onion and celery to the skillet and cook for another 3-4 minutes until the vegetables are softened but still have a bit of crunch.

Make the Sauce: In a small bowl, whisk together the soy sauce, maple syrup, black pepper, white pepper, cornstarch, and water. Pour this mixture into the skillet with the vegetables and cook for 1-2 minutes until the sauce thickens.

Combine and Serve: Once the cauliflower is roasted, add it to the skillet with the sauce. Toss everything together until the cauliflower is well coated with the sauce. Top with finely chopped green onions and a sprinkle of sesame seeds for an added touch. Serve hot and enjoy!