Marinate chicken by mixing with Greek yogurt, ginger, garam masala, turmeric, cumin, red chili powder, and paprika. Cover and refrigerate for at least 30 minutes.

Brown marinated chicken in a skillet over medium-high heat for 3-5 minutes on each side. Transfer to a plate.

In the same skillet, melt butter, sauté onion for 5 minutes, add garlic and ginger for 1 more minute, then add spices.

Combine skillet mixture in slow cooker with tomato sauce and evaporated milk. Add browned chicken and cover with lid.

Cook on low for 6-7 hours or high for 3-4 hours. Stir occasionally if possible.

30 minutes before serving, stir in heavy cream, and cook uncovered to thicken the sauce.

Garnish with fresh cilantro and serve warm with basmati rice or naan.