Preheat your oven to 350 degrees.
Cut the fat cap and any surface silverskin from the racks.
Further French the bones all the way down to the loin to make “spider racks” (how to cut lamb spider racks
Optional: Tie rings of butcher’s twine along the length of the loin between each bone, pulling them tight to compact the meat and form it into a rounder shape.
Doing this will slightly increase your cooking time.
Wrap the bones in a sheet of foil. Season the meat with salt and pepper on both sides.
Get an oiled oven-safe pan hot over high heat. Sear the meat on both sides to develop a crust.
Add the rosemary sprigs and garlic to the pan, then move it to the oven to roast until the lamb is cooked to your liking.
Remove the lamb from the stove and let it rest, loosely covered with a sheet of foil. Reserve 1 sprig of rosemary and 1 clove of garlic from the pan.
Peel & trim the celery root into a cylinder. Trim the potatoes into cylinders. Thinly slice the cylinders into disks that are thin enough to be semi-translucent (using a knife or mandoline).
Get oil hot in a large non-stick pan. Start layering the potato slices in the pan in a rosette pattern, season them with salt and pepper. Cover the potato layer with a layer of celery root, season again, then finish with another layer of potato.
Move the pan to the oven for about 2 minutes, until the bottom of the potato-celery root cake is browned. Flip it and continue to roast until it develops a nice crust on the other side.
Repeat steps 10-11 three more times to produce the rest of the pommes anna cakes (these can be made ahead of time).
Pour both ciders into a frying pan, add the salt, one clove of the garlic from the lamb pan and one of the sprigs of rosemary. Simmer to reduce the cider to a glaze.
Once the glaze has reduced by 3/4, add the butter and whisk to melt it in. Strain the glaze.
Toast the hazelnuts in a dry pan, just until aromatic.
Cut the twine off the racks (if you used it), remove the foil from the bones, and slice the racks into 1-rib chops.
Put a pommes anna cake on each plate, top it with a quarter of the Merino chops.
Drizzle the cider glaze over the top of everything, then sprinkle the plate with the hazelnuts.