Chop the new potatoes into chunks, keeping the skin on, and boil in a pan for 5 minutes.
In the meantime, in a frying pan, melt the butter on a low heat.
Thinly slice the mushrooms and add to the frying pan. Cook for a few minutes.
Add the spinach to the pan and cook until wilted.
Use a colander to drain the water from the boiled potatoes and add to the frying pan with the mushrooms and spinach.
In a bowl, mix the eggs, milk, garlic powder, mixed herbs and ground black pepper using a metal fork.
Pour the egg mix over the vegetables in the frying pan and continue to cook on a low heat for a couple of minutes, or until the bottom of the frittata begins to set.
Place the frittata in the grill for a few minutes to cook the top half.
Cut into segments and serve with a side salad.