Start with marinating chicken in yogurt and 1 tbsp of garam masala for an hour( overnight if possible)
Heat half of the butter and oil in a medium size skillet over medium high heat. Cook chicken until browned and fully cooked, about 5-7 minutes. Set aside
Add another 2 tbsp of butter to the same skillet with chopped onions. and saute until soft. Add minced garlic and cook for another minute.
Add diced tomatoes , garam masala and cooked chicken. Heat it, stir and add heavy cream. Turn the heat off and stir.
Serve over rice and garnish with chopped green onions