Preheat your oven to 425 degrees F. Line a large rimmed baking sheet with parchment paper or foil. You want a hot oven here because that high temperature is what creates the caramelized edges. A lower temperature will just steam the parsnips soft.

Peel the parsnips and cut off the narrow root tip and the thick top end. Halve or quarter the thicker top sections lengthwise so that all your spears are roughly the same thickness, about 1/2 inch wide. Even sizing is the key to even roasting. Spread them in a single layer on the prepared pan, making sure they are not touching each other. Crowded parsnips steam and turn soft instead of caramelizing.

In a large bowl, whisk together the olive oil, brown sugar, rosemary, salt, pepper, and garlic powder if using. Add the parsnip spears and toss well until every piece is coated. Pour them back onto the sheet pan in a single layer. Scrape any sugar and herb mixture left in the bowl onto the parsnips with a spatula.

Roast on the center rack for 20 to 25 minutes, flipping the spears once at the 15-minute mark. They are done when the cut edges are deeply golden brown and slightly caramelized, and a fork slides through the thickest piece without resistance. The tips may darken more than the centers, which is fine and gives good flavor.

Remove from the oven and drizzle the apple cider vinegar over the hot parsnips. Toss gently on the pan. The vinegar cuts through the sweetness and lifts the sticky caramelized bits. Taste and add a pinch more salt if needed. Transfer to a serving platter and serve immediately.