Step 1: Prepare the Oven and Pan - Start by preheating your oven to 350 degrees. While that heats up, line a small rimmed baking sheet with parchment paper. Let the parchment overlap the edges a little so it is easy to lift the frittata out later. Trust me on this part because it makes cleanup unbelievably simple. I learned that lesson after scrubbing baked egg off pans more times than I care to admit.
Step 2: Prep the Vegetables - Slice your mushrooms and green onions before you get started with the eggs. I like cutting the mushrooms into smaller pieces so every bite gets a little bit of everything. Save the green onions for garnish at the end because they stay brighter and fresher that way.
Step 3: Whisk the Eggs - Crack the eggs into a small bowl and add the milk, salt, and pepper. Whisk everything together until the mixture looks smooth and evenly combined. You do not need fancy equipment here. A fork works just fine. The key is making sure the yolks and whites are fully mixed so the frittata bakes evenly.
Step 4: Pour and Layer the Ingredients - Pour the egg mixture into the parchment lined baking sheet. Sprinkle the cheddar cheese evenly across the top first, then add the bacon pieces and mushrooms. Spreading everything out helps each slice get a balanced mix of toppings. Around our house somebody always tries to sneak extra cheese onto one corner of the pan.
Step 5: Bake Until Set - Carefully place the baking sheet into the oven and bake for about 15 minutes. The eggs should look set in the center and lightly golden around the edges. If you gently shake the pan, the middle should not jiggle much. Every oven is a little different, so keep an eye on it during the last few minutes.
Step 6: Garnish and Serve - Once the frittata comes out of the oven, sprinkle sliced green onions over the top while it is still warm. Let it cool for a couple of minutes before slicing into squares. We love serving this on toasted English muffins or thick slices of buttered toast. It turns a simple breakfast into something that feels extra comforting.