We cook the marinade down so none of the flavors go to waste.
Combine 2 tablespoons vinegar and next 5 ingredients (through red onion) in a large ziplock plastic bag. Add 1 serrano chile half to bag; reserve other half for another use. Seal bag; shake well to dissolve sugar. Add pork to bag; refrigerate 2 to 8 hours, turning occasionally.
Heat grill to medium-high (about 450°F). Remove pork and red onion from bag; pat pork dry. Strain marinade into a small saucepan over medium heat; discard solids. Add red onion and remaining vinegar to pan; cook 5 minutes or until liquid is syrupy and reduced by half.
Place pork on grill grate coated with cooking spray; grill 3 minutes. Turn pork; baste top of pork with reduced marinade. Grill 3 minutes or until desired degree of doneness. Place on serving platter; let stand 10 minutes. Slice pork off bone; cut meat into thin slices. Drizzle pork with remaining reduced marinade. Sprinkle with green onions.
VERY disappointed.... this was more like Pork Teriyaki NOT AT ALL Adobo-esque