In a large pot bring 3 cups of water to a boil. Add salt to pot. Add spinach to pot and cook for 30 seconds. Remove spinach from pot and immediately transfer to a bowl with 1 cup of water and ice. When the spinach is cool, drain it and squeeze out the excess water. Chop spinach roughly.
In a large saucepan, heat oil over high heat. Add onions and season with salt and pepper, to taste. Reduce heat to medium-low and saute until it's golden brown. Remove from heat and set aside.
Remove stems and black gills from the mushrooms. In a medium mixing bowl, toss the mushrooms with olive oil and salt and pepper, to taste. Transfer mushrooms to a small baking pan and roast in the oven for 20 to 25 minutes or until tender. Remove from heat and let cool. Once cooled, dice the mushrooms and set aside.
Turn up the oven temperature to 400*F.
In a medium nonstick ovensafe saucepan set over medium-low heat, add 1 teaspoon of butter, 1/2 of the spinach, 1/2 of mushrooms, and 1/2 of eggs. Season mixture with salt and pepper. When eggs begin to set, top with 1/8 cup goat cheese. Place in oven and bake for 5 to 7 minutes.
Repeat with another teaspoons of butter and the other half of the spinach, mushrooms, eggs and another 1/8 cup goat cheese to make a second frittata(seasoning with salt and pepper) and serve. Enjoy!