In a dutch oven, melt 2 tbsp of butter. Add chopped onion and saute until dark brown.
Chop chicken thighs into bite-sized pieces. Place chicken in a Ziploc bag and pour 1 tbsp garam masala, 1 tbsp curry powder, 1 tsp turmeric powder, 1 tsp salt, 1 tsp chili powder. Shake up bag until all chicken is covered in spices.
Line a baking pan with foil and spray with oil. Place the seasoned chicken on the baking tray. Bake on 350 for 25 minutes.
While the chicken is baking, put the remainder of the spices in the dutch oven (1 tbsp garam masala, 1 tbsp curry powder, 1 tsp turmeric powder, 1 tsp salt, 1 tsp chili powder, 1/2 tsp cayenne pepper). Add garlic and saute for 30 seconds.
Add tomato sauce and evaporated milk. Stir until combined. Simmer until chicken is done baking.
Once chicken is baked, put into the sauce. Simmer together for 10 minutes. Serve with basmati rice or naan.