Heat oven to 450° F

On 2 large rimmed baking sheets, toss the cauliflower, Brussels sprouts, and onion with the rosemary and 2 Tbsp of olive oil, and ½ tsp each salt and pepper.

Roast, tossing the vegetables once and rotating the sheets halfway through until golden brown and tender, 15 to 20 minutes.

Add the vegetables, pecorino, and the remaining 2 Tbsp of oil to the vegetables and toss to combine