Wash the potatoes & cook them in a large saucepan of boiling water until partially cooked , 10-12 minutes. Drain them and allow to cool until no more steam rises from them, 10-15 minutes. Using a sharp knife, slice the potatoes.
Heat oil in a large pan/ skillet over medium heat. Add the sliced onions and fry until browned, 10-11 minutes. TO MAKE THE ONIONS SUPER CRISPY, SALT THEM GENEROUSLY while frying. Transfer the onions to a plate lined with kitchen towel.
Add the potatoes to the pan/ skillet & season with salt, dry rosemary and black pepper. Saute over medium heat until the potatoes are well browned on both sides & soft inside. It usually takes 7-8 minutes.
Add back the fried onions.
Garnish with the chopped parsley and serve immediately.
Enjoy Lyonnaise potatoes!