For the cauliflower: Place a baking sheet in the oven and preheat to 425 degrees F. (If your cauliflower is large, preheat 2 baking sheets.)
Whisk the flour together with the garlic powder, onion powder, salt and pepper in a large bowl. Whisk in the milk and 1/4 cup water until smooth. Toss the cauliflower in the mixture until evenly coated. Remove the baking sheet from the oven and spray with nonstick cooking spray. Spoon the cauliflower into the baking sheet, leaving any excess sauce behind in the bowl. (Do not overcrowd the baking sheet or the cauliflower will steam and not become crispy; you may need to bake in batches or use two baking sheets.) Bake the cauliflower for 20 minutes.
For the sesame sauce: Meanwhile, put the soy, honey, sesame oil, vinegar, cornstarch, sesame seeds, ginger, sriracha, garlic and 1/2 cup water in a medium saucepan over medium heat. Whisk together thoroughly. Bring to a boil, then reduce the temperature to medium-low and let simmer, stirring frequently, until the sauce becomes glossy and thick, about 2 minutes. Remove the cauliflower from the oven and toss with half of the sesame sauce. Return the cauliflower to the oven until dark and glossy, 5 to 8 minutes. Remove to a serving bowl and drizzle with more sauce. Add sesame seeds and scallions over the top and serve immediately.