Using a paring knife, remove the half-moon shaped piece of cartiledge from the end of the loin nearest the shoulder, then score the fat just deep enough to penetrate it with a cross-hatch pattern to help it render.
Season the rack all over with salt, pepper and thyme, then place on a sheet of parchment large enough to wrap it up into a package. Apply the bolete mushroom rub, turning the rack over in the powder to help it adhere.
Put the rack bone-side down on the parchment, then sprinkle any remaining mushroom powder over the top. The amount of mushroom rub should seem excessive, but will help form the delicious crust you want.
Wrap the meat in parchment, tying the ends closed to make a package, and refrigerate overnight (optional). To cook the rack of lamb, bake the packet in a pan to catch the fat for 4 hours at 250 degrees, from here you can serve the lamb, although it will be messy. I like to chill to make perfect slices easy, and then reheat when needed.
Make a pan sauce from the drippings (optional)
More options You can use the basic cooking method here for many different cuts of meat. Here's a few ideas.