In a medium-sized, microwave-safe bowl, sprinkle the potatoes with about 1 Tbsp. water, cover them lightly, and microwave them on high heat for 3 – 5 minutes until they are fork tender. (Alternatively, boil or steam them for 5 minutes on the stovetop.)
Preheat the broiler and set the rack 4 – 5 inches from the heat source. Spray a heavy medium-sized, oven-proof skillet with nonstick cooking spray and heat the oil in the same skillet over medium heat.
Sauté the onions until they are softened, about 5 minutes. Add the softened potatoes, any optional fillings, rosemary, salt and pepper and sauté it for about 3 more minutes.
In a separate bowl, combine the eggs, milk and cheese and pour the mixture over the potatoes in the skillet. Raise the heat to medium-high, and cook the mixture for about 2 minutes, until the eggs start to set.
Place the skillet under the broiler and cook it until it is golden brown on top, 3 – 5 minutes. Watch closely so it doesn’t burn. Remove the oven omelet from the oven, let it cool for a few minutes, cut it, and serve it warm (or cold, if you prefer).