2large cooked russet potatoesabout 2 cups quartered then sliced about ¼ inch thick
Heat the oven to 350°F. In a large mixing bowl combine the eggs and cream, whisk until well combined, then add the cheese, parsley, rosemary, salt and pepper. Stir to combine - set aside.
Meanwhile, heat the oil and butter together in an ovenproof skillet, preferably cast iron, over a medium heat; add the onions and sauté, stirring often, until beginning to brown around the edges. Add the potatoes along with a good pinch of salt and freshly ground pepper; cook, stirring gently so as not to break up the potato, until just beginning to brown.
Give the eggs a good whisk, then gently pour around the potatoes. Using a rubber spatula or spoon, stir gently to incorporate the potatoes into the eggs. Cook, lifting the outside edge of the tortilla and allowing the raw egg to slip in under the edge until the tortilla is mostly cooked. Transfer the pan to the top rack of the oven and cook until cooked through about 3 - 5 minutes. Turn on the broiler to high and cook until the top is browned (watch so it doesn't burn). Remove the pan from the oven and allow to rest for 5 minutes. Cut and serve.
LindySez: Another plus of using cast iron pans is heat retention. You could make this Spanish Tortilla a half an hour in advance and it would still be warm. Serve the tortilla hot, warm, or at room temperature.