This beautiful, open-faced sandwich, adapted from one created by Nancy Silverton of Mozza in Los Angeles, uses thinly shaved Manchego cheese to accent the bright green scallion oil, peppery arugula, earthy serrano ham, and runny poached egg. If you can find wild arugula, the smaller leaves are very good on this sandwich.
MAKE AHEAD:
You can poach the eggs ahead of time and slip them into a bowl of ice water. Then just before assembling the sandwiches, warm them in the simmering water for 10 to 20 seconds. The scallion oil can also be made a few hours ahead.