After cutting them all off, I trimmed them up (just a quick slice of the tough stem part and removing any loose outer leaves is all you need to do), slice them in half lengthwise and tossed them with some extra virgin olive oil.
The trick is to start them in a pan, cut side down, so that they can caramelize a bit before you put them into the oven to roast.
I spread them out on a baking sheet along with some diced pancetta that I had fried up until crispy and a sprinkling of grated Parmigiano Reggiano cheese, and roasted them in the oven for about 20 minutes.