Combine broccoli, cauliflower, olive oil, lemon-pepper, seasoned salt, and red pepper flakes in a large bowl; stir well. Spread out evenly on a baking sheet.
Preheat the oven to 400 degrees F (200 degrees C).
Roast in the preheated oven for 12 minutes. Stir and roast until tender, about 12 minutes more.