Preheat oven to 450°. Remove dark green tops from scallions and finely slice; set aside for serving. Finely chop remaining white and pale green parts; transfer to a large bowl. Add egg, garlic, panko, red pepper flakes, 2 Tbsp. oil, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt; mix to combine. Let sit 5 minutes.

Meanwhile, whisk tahini, miso, 1 Tbsp. vinegar, a pinch of sugar, and 2 Tbsp. water in a large bowl to combine. Using oiled hands, gently roll out about 2 Tbsp. portions of chicken mixture into meatballs. Place on baking sheet with reserved green beans.

Bake, turning meatballs once, until green beans are tender and meatballs are cooked through (they will still be very pale), 13–17 minutes.

Meanwhile, whisk soy sauce, ¼ cup vinegar, and remaining 5 Tbsp. sugar in a cold small saucepan to dissolve sugar. Set over medium-high heat and cook, stirring occasionally to keep from sticking to sides, until thick and syrupy, 11–13 minutes. Remove from heat; whisk in remaining 1 Tbsp. vinegar.

Transfer meatballs to a large bowl. Pour 4-5 glaze over and toss to coat. Glaze will thicken as it cools, so toss periodically.

Place green beans on a serving bowl; drizzle with some oil, season with salt, and toss to coat.

Add chicken to bowl with scallion mixture and mix gently to combine.

Divide rice among shallow bowls or plates and top with meatballs and green beans. Spoon any soy glaze remaining in bowl over meatballs. Top with reserved dark green scallion tops.