You'll get a perfectly crispy, golden exterior.
I was nodding enthusiastically before I'd finished reading the title of Kathryn Arthur's article for Heated: "Cast Iron Is the Secret to Foolproof Oven-Roasted Vegetables." Regular readers may know that I'm a big fan of roasting. It's one of my go-to tactics for getting through the large quantities of root vegetables that we receive in a weekly CSA share. I love it because it makes them immediately usable for future meals.
But the specific power of cast iron in creating a glorious caramelized exterior is something I've only recently discovered. It transforms roasted vegetables into an ingredient even more scrumptious than a sheet pan covered with parchment paper ever could. Arthur writes:
"The Maillard reaction is the chemical process that produces the wonderful flavor and beautiful browning on roasted foods. It is definitely possible to achieve this on other pans, but I find cast iron to be much more forgiving than other materials. You’ll still get some decent browning even if you crowded the pan a little more than you should have."
I used to roast vegetables exclusively on sheet pans until I came across a recipe called Tad's Roasted Potatoes in a cookbook by Food52 called A New Way to Dinner. It called for two well-seasoned 12-inch cast iron pans to be filled with diced potatoes, sliced onions, garlic cloves, fresh herb sprigs, and then doused in a generous amount of olive oil, "like you're marinating them." (That line had me salivating.)
The result is a crispy, golden, decadent mass of oily, garlicky potatoes that always leaves me scraping the bottom of the pan for more. Since then I've roasted many more vegetables in cast iron, including carrots, fennel, celery, and sweet potatoes.
Arthur gives some good pointers for using cast iron. You should crank the oven to 425F (or hotter if you're keeping a close eye on it) and preheat the pans so the food sizzles as soon as you add it. Turning on a convection fan helps, too. "[It] circulates the hot air which provides for faster cooking times and better crisping." And, of course, it's better to overcook than undercook.
Cleanup is a bit messier than tossing a sheet of oily parchment, but if you're like me, much of the scraping and nibbling will have already happened at the table. Believe me, it's worth it.