Preheat the oven to 400 degrees F. Set an oven-proof skillet over medium high heat and add one tablespoon of olive oil.
Slice the onions into wedges and separate the petals. When the oil is hot, add the onions to the pan and stir occasionally until they begin to caramelize, 10-15 minutes.
While the onions cook, chop the apples into wedges, leaving the core behind.
Using a vegetable peeler, peel the skin away from the parsnips. Then use the peeler to slice them into ribbons. Stop once you get to the woody core of the parsnip.
When the onions have caramelized, add the apples and one tablespoon of apple cider vinegar to the pan and stir to combine. Turn off the heat and season with salt and pepper.
Pile the parsnip strips on top of the apple mixture, and season with a little more salt and pepper. Place sausages on top and bake for 20 minutes. Flip the sausages over and bake for 20 minutes more.
Use a 12" ovenproof skillet for this recipe, I used my Lodge cast iron skillet. Any smaller and you'll overcrowd the parsnips and they won't brown, any bigger and you might overcook/burn the parsnips and onions.
Parsnips vary wildly in size and shape, so look for some that are the size of large carrots. You don't want a super-thick layer of parsnips or they'll just get mushy, not roasted and browned. This is really a perfectly proportioned recipe. (Believe me, I tried. I wanted more parsnips and tried it with 6, but it just didn't work.)
Depending on the size of your sausages, your cooking time might vary slightly. Just check them at 20 minutes and you should be able to tell.
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