Generously season chicken with salt and pepper, then lightly coat with cornstarch. Set aside for 15 minutes.
Meanwhile, heat the oil in a large saute pan or wak over medium-high heat. Add potatoes and cook until lightly browned on all sides, about 8 minutes. Transfer potatoes to a plate and set aside.
Reduce heat to medium, add onions and ginger to same pan, and cook until soft and fragrant, about 2 minutes. Add turmeric, cook for 30 seconds, then add coconut milk and bring to a simmer.
Add chicken and potatoes and season to taste with salt and pepper. Arrange lemon slices over chicken is tender and cooked through, about 40 minutes.