Mix 2 tbsp oil with garlic, shallots, herbs and mushrooms in a small bowl. Lay out the lamb loins on a prep board, fat side down and season with salt and pepper. Spread the herb/mushroom paste all over the lamb.
Roll up and tie with butcher’s twine, tie in 1″ intervals. Season the lamb with salt and pepper.
Preheat oven to 350 degrees F. In a large cast iron skillet, heat the remaining tbsp of oil until it shimmers, add the lamb loins and cook over moderate heat ,rolling over until browned all over, about 20 minutes in all.
Put the skillet in the oven and roast the loins for 10 minutes, until an instant read thermometer reads 125 degrees in the thickest part. Remove from heat and transfer to a platter and rest for 10 minutes.
Remove string by cutting it off. Carve the loins into 1″ thick slices and serve.