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.

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 grill. 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, 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.