Step 1
Combine 2 garlic cloves, finely grated, 1 lb. skinless, boneless chicken breasts, cut into bite-size pieces, 1 Tbsp. cornstarch, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a medium bowl; season with freshly ground pepper. Toss until chicken is coated (your hands work best here). Set chicken aside.
Step 2
Whisk together 3 Tbsp. gochujang (Korean hot pepper paste), 1 Tbsp. mirin, 1 Tbsp. seasoned rice vinegar, 1 Tbsp. soy sauce, 1 Tbsp. sugar, and 1 Tbsp. water in a small bowl. Set sauce aside.
Step 3
Trim 5 scallions and cut into 1½” pieces; set white and pale green pieces aside. Set dark green pieces aside separately. Thinly slice remaining 1 scallion on a diagonal; set aside for serving.
Step 4
Heat 1 Tbsp. vegetable oil in a large skillet over high until shimmering. Reduce heat to medium-high and add 12 oz. haricots verts or green beans, trimmed, halved crosswise, and reserved white and pale green scallion pieces. Cook, undisturbed, 2 minutes, then toss and continue to cook, undisturbed, until vegetables are charred in spots and tender, 1–2 minutes more. Add one 2″ piece ginger, peeled, finely grated, remaining 4 garlic cloves, finely grated, and reserved dark green scallion pieces; cook, stirring often, until fragrant and scallions have started to char in spots, 1–2 minutes. Transfer green bean mixture to a large plate. If any pieces are stuck to pan or are burnt, add a splash of water and wipe out pan.
Step 5
Heat remaining 2 Tbsp. vegetable oil in same pan over medium-high. Arrange reserved chicken in a single layer and cook, undisturbed, until lightly browned underneath, 3–4 minutes.
Step 6
Reduce heat to medium and return green bean mixture to pan. Add reserved sauce and cook, stirring often, until sauce starts to cling to chicken and green beans, 1–2 minutes. Remove pan from heat and mix 1 tsp. toasted sesame oil into stir-fry.
Step 7
Mound stir-fry on a platter and top with reserved thinly sliced scallion. Serve with steamed white rice if desired.
