1. Prepare the Ingredients

Before you start cooking, make sure you have all your ingredients ready to go. Dice the onion, slice the mushrooms, and chop any additional vegetables or herbs you plan to use. If you’re using any optional ingredients like bacon or sausage, cook them ahead of time and set aside.

2. Sauté the Vegetables

Heat 1 tablespoon of olive oil or butter in an oven-safe skillet over medium heat. Add the diced onion and sauté for 2-3 minutes until softened. Add the sliced mushrooms and bell pepper (if using) to the skillet and cook for 5-7 minutes until the mushrooms have released their moisture and become tender. Stir occasionally to ensure even cooking. If you’re adding garlic, stir it in during the last minute of cooking to avoid burning.

3. Beat the Eggs

While the vegetables are cooking, crack the eggs into a bowl and whisk them until fully combined. If you want a creamier texture, you can add 1/4 cup of heavy cream or milk to the eggs. Season the mixture with a pinch of salt and pepper. You can also add a pinch of herbs like thyme or parsley at this stage for extra flavor.

4. Combine the Egg Mixture with the Vegetables

Once the mushrooms and other vegetables are cooked, pour the beaten eggs into the skillet over the veggies. Use a spatula to evenly distribute the eggs and vegetables. Allow the eggs to cook undisturbed for a minute or two, just until the edges begin to set.

5. Add the Cheese

Sprinkle the shredded cheese evenly over the eggs and vegetables. You can use any cheese you prefer, but cheddar and mozzarella work especially well in this dish. Allow the cheese to melt into the frittata as it cooks.

6. Finish Cooking in the Oven

Preheat your oven to 350°F (175°C). Once the eggs are mostly set (with a soft, slightly runny center), transfer the skillet to the preheated oven to finish cooking. Bake for 10-12 minutes, or until the frittata is golden brown on top and the eggs are fully set. Keep an eye on the frittata to avoid overbaking, as it can become dry.

7. Cool and Serve

Once the frittata is done, carefully remove the skillet from the oven and let it cool for a few minutes before serving. Garnish with fresh herbs like parsley or basil for added color and flavor. Cut into wedges and serve warm.