Heat olive oil in a 10-inch cast iron skillet or other 10-inch ovenproof pan.
Working in batches, lightly fry potatoes and onion until soft, but not browned or crispy. Using a slotted spoon, remove to a bowl or plate. Set aside.
When through frying, reserve 1 tablespoon oil, discard the rest. Rub remaining 1 tablespoon through the bottom and sides to prevent sticking.
Layer potatoes and onions back into the greased skillet.
Pour lightly beaten eggs over the top. The number of eggs will depend greatly on the size of your potatoes. You want the eggs to come up just to the top, if not covering the top, of your potatoes and onions.
Place back onto a hot burner on medium heat until you start to see the sides setting, approximately 3-4 minutes.
Transfer to the oven. Bake for 12-15 minutes, or until the center has just set.
Remove and allow to rest for 1-2 hours, or until ready to serve at room temperature. Garnish with freshly minced parsley, fine sea salt, freshly ground black pepper and finely grated manchego cheese.
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Variations: chorizo, roasted red peppers, sun dried tomatoes, olives, bacon, scallions, chives, chard, kale Use discarded olive oil in salad dressing or other dishes, it is now flavored olive oil!