How to Make It
Brush fish sauce on all sides of short ribs. Grate 2 garlic cloves using a Microplane; place in a small bowl. Add brown sugar to garlic, and stir to combine. Rub garlic mixture on all sides of short ribs. Let stand at room temperature while preparing grill.
Open bottom vent of grill completely. Light charcoal chimney starter filled with briquettes. When briquettes are covered with gray ash, pour them evenly onto bottom grate of grill. Drizzle scallions with oil; toss to coat. Coat top grill grate with oil. Cover and heat grill to medium-high (400°F to 450°F). Place scallions on oiled grate. Grill, uncovered, turning often, until medium-charred, 1 to 2 minutes. Transfer to a cutting board; let cool slightly.
Arrange short ribs on oiled grate. Grill, covered, flipping every 2 minutes, until a thermometer inserted in thickest portion registers 125°F to 130°F, 10 to 12 minutes. Remove short ribs from grill; let stand 5 minutes.
Mince remaining garlic clove; place in a small bowl. Add parsley, orange zest, sumac, kosher salt, and pepper. Rub together with fingers to combine.
Thinly slice grilled scallions. Add to parsley mixture; toss gently to combine. Cut short ribs against the grain into thin slices; sprinkle with flaky sea salt. Serve with gremolata.
If boneless are not available, ask your butcher to slice English-cut short ribs off the bone into rectangular steaks.