Crack the eggs into a large bowl. Finely slice half of the red onion, roughly chop the parsley, and shred the Swiss cheese. Add everything to the bowl. Pour in the milk, season with salt, and mix well.

Heat a skillet over medium heat. Generously butter the pan and sprinkle with a little salt. When hot, pour in the egg mixture. Let it cook gently for 5 to 10 minutes, watching carefully. If using frozen porcini mushrooms, thaw them ahead of time. Finely chop more parsley and thinly slice the remaining half of the red onion. Add the mushrooms, parsley, and onion to the omelet as it cooks.

After about five minutes, fold one third of the omelet over itself to help cook off any excess liquid. Let it cook for another five minutes or so, then serve. Enjoy!