Toss together the beets, Idaho potatoes, sweet potatoes, carrots, fennel, scallions and rosemary in a large bowl. Add the oil and sprinkle with salt and pepper, tossing to combine.
Divide the mixture between 2 rimmed baking sheets and roast, stirring every 10 minutes, until the vegetables have browned slightly and are fork tender, 25 to 30 minutes. Serve on a warmed platter.
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