Pat skinless thighs dry and season with salt and pepper. Heat oil in a large skillet over medium-high heat and brown thighs in 2 batches for 3 minutes per side, then transfer to a 6-quart slow cooker.
Cook onions in the skillet for 6 minutes, then stir in garlic, ginger, cumin and turmeric for 30 seconds. Add broth and scrape up the browned bits.
Pour onions and liquid over the chicken and add preserved lemon rind. Cover and cook on low for 5 to 6 hours until chicken reaches 165°F and is tender around the bones.
Add olives, cover and cook on high for 10 to 15 minutes until hot. Transfer thighs carefully to a serving dish, leaving the bones in the portions.
Skim excess fat from the liquid and mash some softened onions into it with a fork. Stir in lemon juice and spoon the sauce and olives over the 6 thighs.