Preheat oven to 400°F and lightly grease a 9x13-inch baking dish.
Toss cauliflower florets with olive oil, salt, and pepper, then spread on a baking sheet and roast for 20 minutes until tender and lightly golden.
While cauliflower roasts, whisk together Greek yogurt, 3/4 cup cheese, chicken broth, garlic, Dijon mustard, and onion powder in a large bowl.
Add roasted cauliflower and shredded chicken to the yogurt mixture, stir to combine, then transfer to the prepared baking dish.
Top with remaining 1/4 cup cheese and panko breadcrumbs, then bake at 375°F for 25-30 minutes until bubbly and golden on top.