Preheat oven to 450°F. Generously coat a large rimmed baking sheet with cooking spray.
Toss cauliflower, oil, mustard, garlic powder, salt and pepper together in a large bowl until well combined. Whisk flour and milk together in a small bowl until a smooth batter forms. Pour the batter over the cauliflower; toss until evenly coated. Transfer to the prepared baking sheet.
Bake until the batter is lightly golden and the cauliflower is fork-tender, 20 to 25 minutes.
Meanwhile, whisk balsamic glaze and honey together in a small bowl.
Transfer the cauliflower to a large bowl; drizzle with the honey-balsamic glaze and toss to coat. Transfer to a platter and sprinkle with thyme.
