Parboiling skin-on carrots and parsnips lets you soften them without them losing too much moisture. Follow up by browning in a 375°F (190°C) oven for about 40 minutes.
If you can find those cute little baby carrots with the tuft of greens that look like a character from a Disney movie—preferably something fuzzy from Bambi—ought to be chewing on them, then you can cook them whole. Just trim the greens down to a half inch or so (and you can even use the tenderest fronds for garnishing the finished plate).
If all you've got is the big ol' supermarket carrots and parsnips, you'll want to cut them into baby carrot–sized sticks by splitting them lengthwise a couple times, then cutting them into segments. I like to cook mine with their skins scrubbed but intact, because a) it tastes better, b) I'm too lazy to peel 'em, and c) don't you know that rustic is so in these days?
If you try to roast them through in the oven 100% of the way, you end up with shriveled, wrinkled remains. Instead, I find it's much better to par-cook them in salted water, since they'll tenderize without shrinking. This step can be done in advance.
How to Season Roasted Vegetables
Carrots have a natural affinity for spices, or, at least, in my home they do. So I'll generally toss them with olive oil mixed with some variety of spice blend before roasting, whether it's a chili powder (Japanese shichimi togarashi is great), a curry blend, or, in the case of the recipe here, harissa with a touch of cumin and black pepper. Parsnips are more distinctly flavored, and I prefer to keep them simple by just adding some herbs at the end.
After tossing, they go into a moderately hot oven until they're nicely browned on a few sides. It takes about 40 minutes.
After I remove them from the oven, I'll toss my carrots and parsnips with an herb that meshes appropriately with my choice of spices—cilantro goes well with harissa, as does a little pool of crème fraîche to keep things cool.
Parboil quick. Less than 8 minutes. BEWARE
Seasoning was pretty subtle but perfect. Be sure to add salt at the end.