Preheat your oven to 425℉ then heat a 10" skillet to medium heat.

Add a couple of tablespoons of cooking oil to the skillet then saute the diced onion for about two minutes before adding in the sliced mushrooms.

Cook the mushrooms and onions for another 8 minutes or so - until they have released most of their moisture and have begun to brown.

Add the chopped baby spinach to the skillet then stir it into the mushrooms to just wilt it.

When the spinach is wilted, remove the vegetables from skillet and grease it well with butter or oil.

Add the veggies back into the skillet and spread them out evenly.

Beat together the eggs and the milk and season with salt and pepper. Carefully pour the egg mixture on top of the veggies.

Place the skillet back on the burner.  Cook the frittata on the stovetop for about 5 minutes, or until the bottom and edges begin to set and have just begun to firm up then dot the top of the frittata with pieces of the goat cheese.

Put the skillet into the hot oven and bake at 425 for 8-10 mins or until the eggs have set.