Dry the lamb roast off with paper towels. Spread MOST of the Guajillo chile powder on a dish and roll the roast over the chile powder. Encrust all sides of the roast, rather thickly, with the chile powder by pressing the powder into the surfaces of the roast.
Fire up a charcoal grill. Grease the grate to prevent stickage. Grill the lamb over medium-hot coals. Try to form a deep, black crust on all sides of the lamb by turning it infrequently. Meantime, heat up a large skillet over a medium flame. Melt 2 Tbl of butter, throw the onions into the pan with salt and pepper, turn down the flame and caramelize the onions. Check the progress of the lamb periodically. The lamb should be grilled until it reaches rare. The residual heat inside the lamb will carry over and cook it to a perfect medium-rare. Remove the lamb from the grill, salt it down and tent it for about 10 minutes.
Carve the lamb into wafter-thin slices and sprinkle the slices with the chile powder. Throw some tortillas on the the grate of the grill until they puff up, turning once. Assemble the tacos by piling heaps of lamb, onions and strips of chipotle into the grilled corn tortillas. Serve salsa on the side.