Melt butter along with the oil in a pan over medium heat. Add the onion, and saute until soft, about 6-8 minutes.
Add the garlic, ginger, garam masala, turmeric, sweet paprika, and coriander. Continue to toast until fragrant, about 2 minutes. Remove from heat.
Add onions and toasted spices to a food processor or blender, along with the Kosher salt, cashews, Greek yogurt, and tomato sauce. Blend until completely smooth.
Pour sauce into a large bowl and stir in the chicken. Cover with plastic wrap and place your bowl in the fridge to marinate for 6-8 hours.
Place a large pot or skillet with a lid over medium-high heat and add the chicken and all the yogurt marinade. Bring the sauce to a gentle simmer and place the lid on top. Reduce the heat to maintain a simmer, and continue to cook until all the chicken has reached an internal temp of 165 F (about 15-20 minutes).
Serve your korma over a bed of rice with cilantro sprinkled on top, and enjoy!