Peel and cut the potatoes in medium size, not too thick pieces. Slice the Spanish onion.
Cook the potatoes and onions at medium heat, until soft and tender. Add salt to taste.
Remove potatoes from heat. Let them cool slightly.
In the mean time, beat 8 eggs in a large bowl. Add the potatoes and mix well.
Return skillet to the heat and add remaining olive oil.
Pour the egg / potato mixture and cook on one side, at medium-low heat, until the bottom starts to brown.
Place a large plate or a lid on the skillet. Remove from heat and flip the skillet quickly, to turn the omelet around. Slowly slide the uncooked side onto the skillet and return to heat. (Note: Be very careful not to burn yourself while doing this. We recommend using oven mitts or a towel. However, if you do not feel comfortable flipping the omelet this way, place the skillet in the over and bake at 350F until set. Make sure you use an oven proof skillet!)
Prick to top of the omelet with a fork, to allow it to cook a little bit faster. Cook until eggs are no longer runny.