Preheat the oven to 400°F. In a 2-cup glass measuring cup, combine honey, vinegar, red pepper flakes, and 1/2 teaspoon salt. Microwave in 30-second intervals until reduced to about 1/4 cup, stirring between each interval, for about 2 1/2 minutes. Let it cool to room temperature.
In a large skillet over medium heat, heat oil. Season chicken with 2 teaspoons salt and cook skin side down until deeply golden brown, about 10-12 minutes. Flip the chicken and cook for another minute. Transfer to a plate.
Increase heat to medium-high. Add Brussels sprouts and season with remaining salt. Cook until golden brown on the bottom, about 3-4 minutes.
Add scallions to Brussels sprouts and toss to combine. Arrange chicken on top and brush with half of the glaze. Reserve the remaining glaze.
Roast in the oven until vegetables are tender and chicken is golden brown and cooked through, about 8-12 minutes.
Brush chicken with remaining glaze and top with reserved scallion greens before serving.