Preheat the oven to 180°C. Lightly grease a large baking or casserole dish.
Prepare your vegetables. If using firm vegetables such as potato, kūmara, pumpkin, beetroot or carrot, dice them to fairly even sizes.
Spread all of the vegetables and diced cooked chicken evenly through the prepared baking dish.
Sprinkle over the grated cheese.
In a large bowl, whisk together the eggs, cream or milk, garlic powder, onion powder, salt and pepper.
Pour the egg mixture evenly over the chicken and vegetables. Gently move the ingredients around if necessary so the egg mixture works its way through the filling.
Sprinkle with chopped chives, if using.
Bake at 180°C for approximately 1 hour, or until the centre is set and the egg is completely cooked through.
Allow the frittata to sit for 5–10 minutes before slicing and serving.