Slice the rutabaga into 1/8-inch thick slices. (I used a mandoline for this but you can use a really sharp knife - just get the thinnest possible slices you can.) Lay the slices in a single layer on a sheet pan. (You'll need several sheet pans, or do it in batches.)
Whisk together 1 tablespoon olive oil and 1 teaspoon of sherry vinegar in a small bowl. Use a pastry brush to brush this oil mixture over the rutabaga slices. Sprinkle with a half teaspoon salt and a half teaspoon of pepper.
Roast the rutabaga for 7-9 minutes until fork tender. Set aside to cool.
Heat 1 tablespoon of the olive oil in a large sauté pan over high heat (My stove is hot, so I used a medium high heat) until it begins to ripple. Carefully add the onions and sprinkle with 1/2 teaspoon each of salt and pepper. Allow the onions to caramelize for 4-5 minutes, stirring occasionally.
Add raisins. Continue stirring and cooking until the onions have sweated off much of their water and begin to char and raisins plump and take on some color. About 3 minutes.
Remove the pan from the heat and deglaze with the apple cider vinegar. Return pan to the heat and cook until all of the liquid has evaporated, but the onions and raisins still look very wet. Stir in the thyme leaves, remove from heat and set aside to cool.
Whisk together the Dijon mustard and the remaining 1/4 cup olive oil, 1 tablespoon sherry vinegar, 1/2 teaspoon salt and 1/2 teaspoon pepper in a medium bowl. Add greens and pistachios and toss to coat.
To plate -- arrange rutabaga slices in a circle on each plate -- or if serving family style, fan the rutabaga out on a large platter, overlapping slices slightly. Spoon over the onion-raisin mixture and top with greens.
Serve.
Notes
I sprinkled a few more pistachios on top and around the salad -- I just couldn't help myself.