Wash asparagus, break off ends & place on a plate. Mix oil, vinegar, garlic & pepper, then pour the mixture over the asparagus - allow to sit at room temperature for 20 minutes. Cut each slice of prosciutto in half, to make 20 pieces total. Warm a cast iron grill pan for 3-5 minutes over high heat. Wrap each asparagus stalk with prosciutto & place in the grill pan - only cook 10 stalks at a time. Turn the heat down to low & rotate the asparagus frequently with tongs, cooking for 6-8 minutes total.
Note: if you prefer your asparagus to be more fully cooked, consider boiling or steaming for steaming it for 1 minute before marinating.