Preheat oven to 435 F / 225 C then cut up your broccoli, stems and all. I usually trim the bottom 1/8" from the stem, cut it into florets, dividing the florets into 2 or 3 even pieces, depending on their size.
Then cut the stems into 1/4" thick coins.
Put it all in a large mixing bowl.
Toss thoroughly with olive oil and (optional) onion powder.
Spread over two baking sheets/sheet pans. You don't want to crowd the broccoli because then you won't get a good roast with the crispy bits.
Put in the oven and roast for 20 minutes.
If you want to serve with aioli (link above), you can make that now while the broccoli's roasting.
After 20 minutes, take out of the oven, let cool for about 5 minutes, transfer to a serving dish, and watch them disappear!
