Preheat a grill or grill pan to medium heat. Puree the 3 chopped plums in a food processor or blender until smooth. Stir together the pureed plums with the teriyaki, sesame oil, Dijon and sriracha in a large bowl. Add the steak to the bowl, tossing to coat in the marinade, and marinate in the fridge for at least 1 hour.
Cut the remaining 6 plums in half and remove the pits. Thread 6 halves on a long skewer, skewered from top to bottom. Repeat with another skewer.
Remove the steak from the marinade and pat dry.
Add the marinade to a medium saucepan over medium-high heat and bring to a boil. Reduce the heat and simmer until the sauce has reduced slightly, about 5 minutes. Salt and pepper each side of the steak and grill over medium heat on each side for 2
to 3 minutes. Once grilled on both sides, remove from the heat and let rest for 10 minutes.
While the steak rests, put the plum skewers cut-side down on the grill and cook for 2 minutes. Flip and grill the other side for 2 minutes. Brush the plums with the marinade, flip and cook for an additional 1 to 2 minutes.
Remove the plums from the skewers onto a platter. Slice the skirt steak against the grain and brush some marinade on it. Serve with scallions on top.