Peel the Parsnips and using a mandolin, slice parsnips to 1/8 thin slices. The thinner the slices the faster this will bake. If you don’t have a mandolin, slice them as thinly as humanly possible! 😉 Thinly slice the onion and grate the cheese.
Generously grease a 9 x13 inch baking dish with butter.
Layer the parsnips and onion with 2/3 of the cheese and ⅔ of the thyme -saving the rest for the top- and reserving the more perfect pieces of parsnips for the final top layer. Press down and make sure layers are even. It’s OK if the inside layers are messy. Place the last layer of parsnip slices in nice looking overlapping rows.
In a small pot, heat 3 Cups heavy whipping cream. Whisk in salt, nutmeg, the remaining thyme, white pepper, flour and the garlic and when it just comes to a simmer, pour over the layered parsnips, slanting the baking dish to get the cream in all the nooks and crannies. Sprinkle the remaining cheese over the top, a few more leaves of thyme and a little sprinkling of nutmeg. Cover securely with a lid or foil and bake for 50-60 minutes on the middle rack until parsnips are fork tender. Check, Remove foil, pierce with fork, parsnips should be al dente. (If not- your parsnips were perhaps cut thicker that 1/8 of an inch, and you will need to cook longer with the foil on.) Also, don’t worry if it seems watery at this point.
Once fork tender, remove foil and bake another 15 minutes uncovered , or until golden and bubbly. It is important to let it sit at room temp for 15-20 minutes before serving, so it sets up and thickens.
You can also make this all ahead, and simply reheat before serving.
You can also substitute half of the parsnips with potatoes.
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