Peel and cut root vegetables into similar size pieces (1.5"- 2" inch pieces work well)
If making ahead, divide vegetables (omitting leeks) into 2 large freezer plastic bags (1 gallon size). If preparing the vegetables right before cooking, combine them all in a large mixing bowl.
Combine oil, garlic and rosemary and pour over vegetables. Season liberally with salt and pepper. Toss to combine. If making ahead, distribute oil combination evenly between bags of vetebales, season, seal and shake bags slightly to distribute oil and seasoning throughout.
Bagged vegetables can be stashed in the refrigerator for up to two days.
To roast, preheat oven to 400 degrees.
Spread root vegetables evenly onto 2 large baking sheet trays. Do not crowd vegetables and avoid including any juices that have accumulated- they will cause the vegetables to steam rather than roast.
Roast in hot oven for 30 minutes. When vegetables are starting to brown lighlty and softening, toss leeks in a teaspoon of oil and add them on top of root vegetables.Roast another 15 minutes or until vegetables are browned around the edges and softened throughout.
Remove from oven and check for seasoning. If needed, sprinkle additional salt and pepper immediately after removing from the oven.
Leftovers can be stored in the refrigerator for up to 5 days and are wonderful in tossed salads.