Begin by cutting the chicken thighs into bite-sized pieces. Season lightly with salt and pepper.
In a skillet over medium heat, add the vegetable oil. Once hot, add the diced onion and sauté until translucent, about 5 minutes.
Add the minced garlic and grated ginger to the skillet. Sauté for an additional 2 minutes until fragrant.
Stir in the curry powder, cumin, coriander, turmeric, and cayenne pepper. Cook for 1 minute, mixing well to incorporate the spices.
Transfer the sautéed onion and spice mixture into your slow cooker. Add the chicken pieces, coconut milk, and diced tomatoes. Stir well to combine all ingredients.
Cover the slow cooker and set it on low for 6 hours, or high for 3 hours, until the chicken is tender and cooked through.
About 15 minutes before serving, stir in the spinach until wilted. Adjust salt and pepper to taste.
Serve the curry over rice or with naan, garnished with fresh cilantro, if desired.