Clean and top the beets and, along with the fingerling potatoes, drizzle with enough olive oil just to coat. Roast whole at 400 degrees until softened, but not completely cooked (about 30 minutes). Remove from the oven and carefully peel the beets, then cut in half. You can leave the fingerlings whole, and slice lengthwise in half before serving, or halve them and continue roasting (both ways are delicious!)
Quarter the fennel and red onion lengthwise, and cut the kabocha into 1/2 inch wedges.
Toss all the vegetables in a bowl and drizzle enough olive oil to sufficiently coat. Then, add a big splash of balsamic vinegar (approx. 1/4 cup), a couple hearty pinches of Kosher salt, a few good turns of fresh ground black pepper, and a small sprig of fresh rosemary. Let roast another 30-40 minutes, turning every so often to get an even roast.