I always start by setting my oven to 425°F and letting it fully heat up before anything goes in. A properly hot oven is the secret to that caramelized, crispy exterior instead of a soft, steamed one. While it heats, I line my baking sheet or just lightly oil it so nothing sticks later.
I cut all my root vegetables into roughly 1-inch chunks, trying to keep the sizes consistent. This step matters more than people think because uneven pieces cook unevenly, leaving some mushy and others still hard in the middle by the time you pull the tray out.
In a large bowl, I combine the chopped vegetables, red onion, and garlic with olive oil, salt, and pepper. I use my hands to really massage the oil into every piece so nothing roasts dry. This is also when I decide if I want extra spices that day.
I spread everything out on my baking sheet in one even layer, making sure pieces aren't piled on top of each other. If your tray looks crowded, I'd rather use two trays than crowd one, since crowding is the number one reason roasted veggies turn soft instead of crispy.
I slide the tray into the oven and let it roast undisturbed for about 30 minutes. This gives the vegetables time to start browning on the bottom before I touch them again, which really locks in that caramelized flavor everyone loves.
I pull the tray out, sprinkle on the chopped rosemary, and give everything a good toss so the herbs coat every piece. This is also the point where I check for seasoning and add a touch more salt if needed.
Back into the oven it goes for another 10 to 15 minutes, until the vegetables are fork-tender and deeply golden at the edges. I like to peek at the 10-minute mark since ovens vary and I'd rather check early than risk burning.
Once out of the oven, I let the tray rest for a couple of minutes, then drizzle balsamic vinegar over the top if I'm using it. I give it one final toss and serve it warm, straight from the pan.