Pre-Heat oven to 180 degrees Celcius.
Heat butter/oil in a Cast Iron/Oven proof pan. Add onions and saute for 5 minutes till they are soft and translucent.
Add chopped ginger and green chillies and cook for another minute. To this, add mushrooms and cook till all the water from the mushrooms reduces.
Mix in chopped bell pepper, turmeric powder, garam masala, black pepper, coriander leaves and salt.
Meanwhile, in a separate bowl crack eggs. Add milk and season it with salt and pepper (keep in mind that the onion mixture also has salt and pepper). Beat well.
Reduce the flame to medium-low heat and add the egg mixture slowly to the pan. Shake the pan to spread the mixture evenly.
Let this cook for 5-6 minutes on a low heat undisturbed.
You can either continue to cook the Frittata on the stove top till the center is set (about 10-12 minutes) or place the pan on the middle rack of the oven and cook for 20-25 minutes.
Serve this One Pan Indian Masala Omelette Frittata with some tomato ketchup or hot sauce drizzled on top.