Preheat oven to 425°F. Line a large rimmed baking sheet with parchment paper.
Whisk miso, water, cornstarch, sesame oil, chili sauce and garlic together in a medium bowl until smooth. Add cauliflower; toss well to combine. Spread on the prepared baking sheet and coat with cooking spray. Roast until tender and slightly charred, stirring halfway through, about 30 minutes. Garnish with scallions, if desired.