Preheat oven to 375F and place parchment paper in a large baking sheet with a lip to line the entire pan.
Heat olive oil in a large pan over medium heat and cook the onions for about 5 minutes. Add in the mushrooms and spinach and cook another 5 minutes.
Whisk eggs in a large bowl and add the vegetables to the eggs. Season with garlic powder, salt and pepper.
Pour egg mixture into prepared baking sheet to make a thin layer and bake for 20-30 minutes or until eggs are fully cooked.
Broil English muffins with a slice of cheese on each in oven for a few minutes or until toasted and cheese melts.
As the muffins are getting crispy, use a glass about the same size as the muffins and cut the eggs to fit them.
Top the bottom English muffin with the egg circle and some of the leftover pieces of egg and then add tomato and avocado and the top half of the muffin.
*These can be frozen by assembling without the avocado and tomato, wrapping in foil and then putting it in a freezer bag and placing in the freezer. When ready to eat, remove from foil and microwave for 60-90 seconds or place in the toaster until cooked through. Top with avocado and tomato and enjoy.