Using flat skewers helps the meat char and cook evenly by curbing any rolling around the grill. Be sure to preheat the grill; high, even heat will help guarantee strong grill marks and will caramelize the sugars in the marinade.
How to Make It
Cut skirt steak crosswise (with the grain) into 8-inch-long pieces. Working with 1 piece at a time, cut lengthwise (against the grain) into 1-inch strips. (If any of the pieces are thicker than 1/4 inch, carefully butterfly thick pieces crosswise and open up to create larger 1/4-inch-thick pieces.) Set aside.
Stir together cilantro, beer, garlic, oyster sauce, sugar, fish sauce, lemongrass, and black pepper in a large bowl. Add steak, and toss to coat. Let stand at room temperature 15 minutes, tossing occasionally.
Remove steak from marinade, and thread onto skewers, folding back and forth, accordion-style. Transfer skewers to a rimmed baking sheet. Cover and chill until ready to use (up to 4 hours).
Open bottom vent of a charcoal grill completely. Light 2 charcoal chimney starters filled with briquettes. Let charcoal burn until you see white rings around the charcoal on the top of the chimney, (there can still be flame). Pour briquettes evenly onto bottom grate of grill. Adjust vents as needed to maintain an internal temperature of 500°F to 550°F. Coat top grill grate with oil; place grate on grill.
Season skewers all over with salt. Place skewers on oiled grates; grill, uncovered, turning occasionally, until charred and cooked to desired degree of doneness, 6 to 8 minutes for medium. Transfer skewers to a serving platter. Garnish with microgreens or scallions.