Heat oven to 425°F; toss cauliflower with 2 tablespoons oil, garlic powder, salt, and pepper on large sheet.
Roast 25-30 minutes, stirring halfway, until golden and tender.
Meanwhile, heat remaining tablespoon oil in large pot over medium; cook onion about 5 minutes until softened.
Cook garlic another minute until fragrant; add thyme.
Add roasted cauliflower and broth; simmer 10 minutes.
Immersion-blend completely smooth and creamy, or carefully transfer to regular blender in batches.
Add almond milk, nutmeg, and optional nutritional yeast. Adjust salt and pepper; garnish with herbs and serve warm.