Preheat oven to 350°F. Pat chicken dry and season generously with kosher salt.
Heat oil in a large oven-safe skillet over medium-high. Sear chicken skin side down until golden, about 5 minutes. Flip and cook another 4 minutes. Transfer to a plate.
Remove pan from heat and let fat cool for 2–3 minutes. Return to medium heat, add onion, and cook 6–8 minutes, stirring and splashing in water if needed, until softened and golden.
Add garlic and cook for 1 minute. Stir in turmeric and red pepper flakes and cook another 30 seconds.
Pour in vinegar and chicken stock. Season lightly with salt and bring to a simmer.
Nestle chicken into the sauce, skin side up, and add blood orange halves cut side down.
Transfer pan to oven and bake uncovered for 30–35 minutes, until chicken is fully cooked and sauce is reduced.
Transfer chicken to a serving platter. Spoon sauce over top and squeeze blood orange halves for a final acidic hit. Finish with a drizzle of avocado oil if desired.