Step 1Preheat oven to 450°. Remove dark green tops from 3 scallions and thinly slice; set aside for serving. Finely chop remaining white and pale green parts; transfer to a large bowl. Add 1 large egg, 2 garlic cloves, finely grated, ½ cup panko, ¼ tsp. crushed red pepper flakes, 2 Tbsp. extra-virgin olive oil, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt; mix to combine. Let sit 5 minutes.
Step 2Meanwhile, whisk 1 (heaping) Tbsp. tahini, 1 tsp. miso, 1 tsp. unseasoned rice vinegar, a pinch of sugar, and 2 Tbsp. water in a large bowl to combine. Set tahini dressing aside.
Step 3Place 1 lb. green beans, trimmed, on a rimmed baking sheet; drizzle with some oil, season with salt, and toss to coat.
Step 4Add 1 lb. ground chicken to bowl with scallion mixture and mix gently to combine. Using oiled hands, scoop out about 2 Tbsp. portions and roll into golf-ball-size meatballs. Place on baking sheet with green beans.
Step 5Bake, turning meatballs once, until green beans are tender and meatballs are cooked through (they will still be very pale), 13–17 minutes.
Step 6Meanwhile, whisk ⅓ cup soy sauce, ⅔ cup unseasoned rice vinegar, and remaining 5 Tbsp. sugar in a cold small saucepan to dissolve sugar. Set over medium-high heat and cook, whisking occasionally to keep from sticking to sides, until thick and syrupy, 11–13 minutes. Remove from heat; whisk in remaining 1 Tbsp. unseasoned rice vinegar.
Step 7Transfer meatballs to a large bowl. Pour soy glaze over and toss to coat. Glaze will thicken as it cools, so toss periodically to build up coats of glaze on meatballs until ready to serve.
Step 8Transfer green beans to bowl with reserved tahini dressing; add sesame seeds as desired and toss to coat.
Step 9Divide steamed rice among shallow bowls or plates and top with meatballs and green beans. Spoon any soy glaze remaining in bowl over. Top with reserved dark green scallion tops.