Preheat oven to 375°F.
In a bowl, whisk together the orange juice, soy sauce, mango puree, garlic, onion, ginger, rice vinegar, and brown sugar. Set aside.
Pat the chicken drumsticks dry with paper towels.
Season lightly with kosher salt and black pepper.
Coat evenly with cornstarch.
Heat olive oil in a large oven-safe skillet over medium-high heat.
Sear the chicken on all sides until lightly golden, about 6–8 minutes total.
Remove chicken from the skillet and set aside.
Reduce heat to medium-low.
Pour the sauce mixture into the skillet, scraping up any browned bits from the bottom of the pan.
Simmer for 5–7 minutes until slightly thickened.
Stir in the orange zest, diced mango, and bell peppers.
Return the chicken to the skillet.
Cover and bake for 20 minutes.
Uncover and continue baking for an additional 20 minutes, or until the chicken reaches an internal temperature of 165°F.
Transfer the skillet back to the stovetop over low heat.
Simmer for 5–10 minutes, spooning the sauce over the chicken frequently, until the sauce becomes thick, glossy, and sticky.
Garnish with sliced scallions and sesame seeds, if desired.
Serve warm.