Heat 1 tablespoon butter in an 8-inch non-stick skillet over medium-high heat until melted and foaming. Add sliced mushrooms in a single layer, season with salt and pepper, and cook 5-7 minutes without stirring often until golden brown and moisture evaporates. Transfer mushrooms to a plate.
1 tbsp butter, 4 oz mushrooms, to taste salt, to taste black pepper
Crack 2-3 eggs into a bowl. Add 1 tablespoon milk or cream if using, salt and black pepper to taste. Whisk vigorously for 30 seconds until frothy and uniform with no streaks of white.
2-3 large eggs, 1 tbsp milk or cream, to taste salt, to taste black pepper
Reduce heat to low-medium. Add a small pat of butter to the skillet, swirl to coat. Pour in whisked eggs and cook undisturbed for 2-3 minutes until edges set and pull away from pan but center remains slightly wet and shiny.
1 tbsp butter, 2-3 large eggs, 1 tbsp milk or cream
Sprinkle shredded Swiss cheese over one half of the omelet, top with the cooked mushrooms. Use a spatula to gently fold the empty half over the filling. Cover the pan, turn off heat, and let sit 30 seconds to melt the cheese with trapped steam.
2 oz Swiss cheese, 4 oz mushrooms
Run spatula around edges to loosen. Tilt the pan and slide the omelet onto a plate in one smooth motion. The underside should be light golden with no brown spots; the inside remains slightly creamy. Let rest 1 minute before eating.
