Preheat oven to 400ºF. While oven is heating, pat chicken thighs dry with a paper towel and season generously with salt and black pepper.
Heat olive oil in an ovenproof skillet over medium-high heat and add thighs skin side down. Cook until nicely browned, about 6 to 8 minutes. Reserve a generous tablespoon of fat from the skillet. Turn the chicken thighs skin side up and transfer skillet to the oven. Reduce oven heat to 375ºF and roast chicken for 20 minutes.
While the chicken roasts, make the glaze. Heat the reserved chicken fat to a medium saucepan over medium-high. Add the onions, season with salt and pepper, and cook for 3 to 5 minutes, stirring frequently, until softened. Add the ginger, crushed red pepper and 1/2 cup of water, bring to a gentle boil and reduce the heat to medium.
Scrape up any browned bits on the bottom of the pan with a wooden spoon. Add the orange juice, honey and tamarind, and simmer for 5 to 7 minutes, until glossy and reduced. As it simmers, taste to see if it needs more tamarind—add just a little at a time until you get the right balance of sour and sweet.
Pour the glaze over the chicken and return to the oven to roast for an additional 12 to 15 minutes, until the skin is caramelized and the sauce is slightly thickened.
Serve with rice and garnish with the orange zest (if using), scallions and mint.