When using this recipe as part of a tapas presentation, it should be cut into very small squares or triangles. The frittata can be made a day ahead and served at room temperature.
Preheat oven to 375°. In large, ovenproof skillet over low heat, cook onions in butter and oil until soft. Raise heat and add ham, cooking an additional 2-3 minutes. Set aside to cool.
In large bowl, beat eggs with parsley, cheese, milk, salt and pepper. Add ham and onion mixture. Peel and grate potatoes, using largest hole on grater. Squeeze all liquid from potatoes and add to egg mixture. Heat remaining 2 TBS butter in skillet. When melted, pour in egg mixture. Place skillet in oven and bake for 30-40 minutes, or until the top is golden brown. 6 servings.