Add 2 Tb butter to skillet and sauté chopped onions until caramelized.
Stir in tomato paste, garlic paste, and ginger paste. Cook until fragrant.
Mix spices (paprika, cayenne, garam masala, cumin, coriander, salt, pepper) and add to pan.
Pour in crushed fire roasted tomatoes and water, scraping pan to deglaze.
Transfer to blender and blend until smooth.
Add diced chicken to slow cooker and cover with sauce.
Cook on high 2–3 hours or low 3–4 hours.
Stir in 2 Tb butter, then add Greek yogurt. Let sit 10–15 minutes before serving.
