Preheat the oven to 425˚F/220˚C (or 450˚F/230˚C if your oven is clean, and use convection at either temperature if you have it).
Cook the bacon over medium heat on the stovetop until the fat has rendered and the bacon is tender but not crisp, about 15 minutes (this can be done in a skillet or directly in the roasting pan you will use for the vegetables).
Combine the bacon and rendered fat with the parsnips, beets, celery root, potatoes, and carrots in a roasting pan, stirring well to coat the vegetables in fat. Roast for 20 minutes. Add the onion, garlic, and 2 or 3 generous pinches of salt to the pan and stir to distribute the onion and garlic. Continue to cook until the vegetables are browned and tender, another 40 minutes or so (feel free to give them another stir midway through this final cooking).
Remove the pan from the oven and give the vegetables a generous grinding of black pepper. Sprinkle the vinegar over the top, followed by the olive oil. Toss, and then serve immediately.