In a large ziplock bag, combine chicken thighs with Greek yogurt, minced ginger, garam masala, turmeric, cumin, red chili powder, and paprika. Seal and massage the marinade into the chicken. Refrigerate for at least 30 minutes or overnight.

In a skillet over medium-high heat, add oil and heat up. Cook the marinated chicken thighs for about 3-5 minutes per side until golden brown. Remove the chicken from the skillet and keep warm.

In the same skillet, melt unsalted butter over medium heat. Add diced onions and minced garlic, sautéing for 2 minutes until onions are translucent. Stir in minced ginger and cook for another minute before adding spices.

Transfer the sautéed onion and spice mixture into your slow cooker. Add the browned chicken thighs, pour in tomato sauce and evaporated milk, and season with salt and pepper. Stir to combine.

Cover the slow cooker and set to cook on low for 6-7 hours or high for 3-4 hours. Check for doneness near the end of cooking time.

About 30 minutes before serving, stir in heavy cream for a rich finish. Serve over jasmine rice or with naan.