Preheat oven to 425°F and line a large baking sheet with parchment paper.
Brush cauliflower steaks with olive oil and season with garlic powder, salt, and pepper. Roast for 20 minutes, flipping halfway through, until golden and tender.
While cauliflower roasts, mix shredded chicken with buffalo sauce in a medium bowl until well combined.
Remove cauliflower from oven and top each steak with buffalo chicken mixture, then sprinkle with mozzarella cheese.
Return to oven and bake for 8-10 minutes until cheese is melted and bubbly.
Top with crumbled blue cheese, sliced green onions, and drizzle with ranch dressing before serving.