In a small bowl, stir together the hoisin, sherry and soy sauce; set aside. In a 12-inch skillet over medium-high, heat the oil until shimmering. Add the bell peppers and cook, stirring occasionally, until softened and beginning to brown, 4 to 5 minutes. Add the garlic, ginger and pepper flakes, then cook, stirring, until fragrant, 30 to 60 seconds.
Add the chicken and cook, stirring occasionally, until lightly browned and opaque throughout, 7 to 8 minutes. Add the hoisin mixture and cook, stirring, until the sauce slightly thickens, 1 to 2 minutes. Off heat, taste and season with additional soy sauce, if needed.
