Preheat oven to 350°F. Spray 4 cups in a muffin tin with cooking spray.
Whisk eggs together in a bowl. Dice mushrooms and onion, and chop spinach.
Add butter to a medium skillet and melt over medium heat.
Add mushrooms and onions to skillet and cook until slightly softened, stirring frequently. Stir in spinach, then divide mixture equally among prepared muffin cups.
Evenly distribute eggs on top of the veggies (filling cups about 3/4 full).
Bake for 15-20 minutes or until eggs are cooked through.
Split each English muffin in half; add shredded cheese and an egg cup to each bottom half, lightly pressing down on the egg to flatten if necessary.
Top with remaining muffin halves and enjoy with kiwi slices on the side.