Whisk together garlic and ¾ cup vinegar in a large bowl. Season steak all over with 1 Tbsp. salt and 1 Tbsp. sugar; transfer to marinade and turn to coat. Let sit, turning occasionally, 10–15 minutes.
Prepare a grill for medium-high heat; oil grate. Grill scallions (making sure to arrange them perpendicular across grates so they don’t fall through), turning occasionally, until charred and softened, 6–8 minutes. Transfer to a cutting board and let cool slightly.
Grill steaks until charred and an instant-read thermometer inserted into the thickest part registers 120°, 3–4 minutes per side for medium-rare. Transfer to cutting board and let rest 10 minutes.
Meanwhile, trim roots from scallions and thinly slice. Transfer to a medium bowl and add chiles and remaining 1¼ cups oil, 3 Tbsp. vinegar, 1 Tbsp. salt, and 2 tsp. sugar and mix well to combine. Taste and season with more salt if needed.
Slice steak against the grain into ¼"-thick strips. Arrange on a platter and spoon some scallion salsa verde over. Serve additional salsa verde alongside.