A few minutes prior to grilling, dry off and salt down the eye of round. Roll it in a sizeable portion of the Guajillo chili powder. Verify that the surfaces are completely encrusted by the rub.
Grill the roast over medium hot coals, turning infrequently, until it is medium rare. (At medium rare, this roast will achieve its most succulent and tender texture.) Do not fear the flesh blackening, as that is the point behind this procedure.
Allow the roast to rest 10 minutes before carving. Carve it into wafer-thin slices.
Warm the flour tortillas over the hot coals. Pile heaping slices of the roast, a few pearl onions and strips of chipotle peppers into the tortillas. Fold up burrito style, topping with salsa and sour cream.