Heat the oven to 400°F/200°C.
Toss the sweet potatoes with 2 tablespoons olive oil, the garlic powder, 1 teaspoon salt and the pepper on a sheet pan. Spread in an even layer and roast until beginning to soften, 15 minutes. Turn the potatoes.
Add the asparagus or green beans to the pan. Nestle the chicken thighs skin-side up among the vegetables. Drizzle with the remaining olive oil and season lightly with salt.
Whisk the miso, honey, coconut aminos or soy sauce, rice vinegar, ginger, garlic and a pinch of salt until smooth. Spoon over the chicken, reserving a few tablespoons for finishing. Roast until the chicken is deeply browned and cooked through and the vegetables are tender, 35 to 40 minutes.
Brush the reserved glaze over the hot chicken and vegetables. Transfer to a platter or serve from the pan. Finish with sesame seeds and scallions and serve immediately.