Preheat oven to 400°. Set racks at medium high and medium low.
Line two baking sheets with parchment paper.
Clean all vegetables well.
Peel carrots, parsnips, sweet potatoes and onion.
Cut sweet potatoes into 3/4 inch thick slices and quarter.
In one bowl, toss sweet potatoes with enough olive oil to coat (about 1 Tbsp), a pinch of salt and pepper (more or less to taste). Set aside.
Cut potatoes into 3/4 inch thick slices and quarter.
In a separate mixing bowl, toss potatoes with enough olive oil to coat (about 1 Tbsp), the rosemary, and a pinch of salt and pepper (more or less to taste). Add
Pour both the sweet potatoes and regular potatoes onto one baking sheet and spread out in one single layer. The less potatoes touch, the better.
Put the potatoes in the oven on the higher rack. Roast for 40-45 minutes. They are done when they have browned on the outside and are soft inside when pierced with a fork.
Cut parsnips and carrots into 3/4 inch pieces, leaving thin ends longer to avoid small pieces that burn. Toss with enough olive oil to coat (about 1 Tbsp), a pinch of salt and pepper (more or less to taste).
Cut onions in half and trim the root end, leaving just enough to hold the bulb together. Cut each half into six wedges.
In a small bowl, gently toss the onions with enough olive oil to coat (about 1 Tbsp), a pinch of salt and black pepper (more or less to taste).
Pour the carrots, parsnips and onions onto one baking sheet lined with parchment paper, spread out in one layer leaving space between pieces as you can.
Put this pan of vegetables into the oven on the lower rack. Roast for 30-35 minutes. They are done when the carrots and parsnips are lightly brown, and are soft inside when pierced with a fork. The onions will be dark brown and soft.
If you do not have a convection oven, stir the vegetables on each pan and rotate pans halfway through roasting. If you have a convection oven, rotate pans only if you have a hot spot that cooks more quickly.
