Trim the ends of the brussels sprouts, and rinse in a large bowl of cold water. Transfer to a baking sheet and pat dry. Halve the sprouts. Place in a large bowl, toss with olive oil and season well with salt and pepper. Transfer the sprouts to a sheet pan and roast in the oven for 30 to 40 minutes, shaking the pan occasionally to brown the vegetables evenly, until crisp on the outside and tender within. Sprinkle a bit more salt and red-pepper flakes.