Step 1: Preheat and Prepare Your Pan
First things first, preheat your oven to 350°F (175°C). This is a moderate temperature that cooks the eggs gently, preventing them from becoming rubbery. Next, and this is crucial, thoroughly grease your 12-cup muffin tin. Use a good quality non-stick cooking spray, making sure to coat the bottom and all sides of each cup. Alternatively, you can use a pastry brush to coat each cup with melted butter or coconut oil. Do not skip or skimp on this step, as eggs are notorious for sticking.

Step 2: Sauté the Vegetables
Heat the olive oil or butter in a large skillet over medium heat. Once shimmering, add the finely diced onion and red bell pepper. Sauté for 3-4 minutes until the onion becomes translucent and the pepper starts to soften. Add the chopped mushrooms and continue to cook for another 5-7 minutes. The mushrooms will release a lot of water; it’s important to cook them until this moisture has evaporated from the pan. This is the key secret to preventing watery egg muffins.

Step 3: Wilt the Spinach
Once the other vegetables are cooked and the pan is relatively dry, add the two cups of fresh baby spinach. It will seem like an enormous amount, but don’t worry. Stir it continuously with your spatula, and it will wilt down to a fraction of its original size in just 1-2 minutes. Once wilted, remove the skillet from the heat and set it aside to cool slightly.

Step 4: Prepare the Egg Mixture
While the vegetables are cooling, crack all 12 large eggs into a large mixing bowl. Add the 1/4 cup of milk or cream, salt, black pepper, and optional garlic powder. Using a whisk, beat the mixture vigorously for about a minute. You want the yolks and whites to be completely combined and for some air to be incorporated. This aeration is what helps make the muffins light and fluffy.

Step 5: Combine the Ingredients
Once the sautéed vegetables have cooled for a few minutes (so they don’t scramble the eggs), add them to the whisked egg mixture. Add 3/4 cup of the shredded cheddar cheese, reserving the final 1/4 cup for the topping. Gently fold everything together with a spatula until the vegetables and cheese are evenly distributed throughout the egg base.

Step 6: Fill the Muffin Cups
Carefully pour or ladle the egg mixture into your prepared muffin tin. Fill each cup about three-quarters of the way full. Leaving a little space at the top is important, as the eggs will puff up significantly while baking. Overfilling will result in a messy overflow in your oven.

Step 7: Top and Bake
Sprinkle the remaining 1/4 cup of shredded cheese evenly over the top of the mixture in each muffin cup. This will create a delicious, golden-brown cheesy crust. Place the muffin tin on the center rack of your preheated 350°F (175°C) oven.

Step 8: Bake to Perfection
Bake for 20-25 minutes. The muffins are done when they have puffed up, are set in the center, and the edges are lightly golden brown. To check for doneness, you can insert a toothpick or a thin knife into the center of a muffin; it should come out clean, with no wet egg mixture attached.

Step 9: Cool and Remove
Remove the muffin tin from the oven and place it on a wire cooling rack. Let the muffins cool in the tin for at least 5-10 minutes. They will deflate as they cool—this is completely normal! As they cool, they will also pull away from the sides of the pan slightly. To remove them, gently run a thin knife or an offset spatula around the edge of each muffin to loosen it, then lift it out.