Heat oven to 400 degrees. Toss the carrots with 2 tablespoons oil, and season lightly with salt. Spread in an even layer on 2 baking sheets and roast until vegetables are tender and caramelized in spots, about 40 minutes, rotating sheet pans once halfway through the cooking process.
As the carrots roast, make the relish: Grate the ginger into a large bowl and add the sliced scallions. Zest 1 lemon over the bowl, and squeeze in the juice from 1½ lemons. (Reserve the other half for serving.) Stir to combine. Add the remaining 2 tablespoons oil. Stir in the roasted peanut oil (if using), the yaji spice, paprika and parsley. Stir to combine,then taste and adjust the seasoning if necessary.
While the vegetables are still warm, gently toss in the large bowl to coat with the relish. Transfer to a platter, and scatter the toasted peanuts over the top. Serve with reserved lemon half, if you like.