Preheat oven to 400°F and lightly grease a 9x13-inch baking dish with 1 tablespoon olive oil.
Toss cauliflower and broccoli florets with 1 tablespoon olive oil, salt, and pepper, then spread on a baking sheet and roast for 15 minutes until slightly tender.
While vegetables roast, drain cashews and blend them with plant-based milk, nutritional yeast, garlic, onion powder, salt, and pepper until completely smooth and creamy.
Transfer roasted vegetables to the prepared baking dish and pour cashew cream sauce evenly over the top, stirring gently to coat.
Sprinkle panko breadcrumbs over the casserole and bake at 375°F for 20 minutes until golden brown and bubbly.
Remove from oven, let rest for 5 minutes, garnish with fresh parsley, and serve warm.