Step #2 Halve the potatoes lengthwise & cut each half into 1/4-inch-thick slices.
Step #3 Heat the oil in an ovenproof, preferably nonstick, skillet over med-heat/flame.
Step #4 Add the potatoes & cook this until almost tender, about 10 mins.
Step #5 Add the onion & cook this until softened, about 5 mins.
Step #6 Carefully pour off all but 1 tbsp of the oil from the potatoes & onion.
Step #7 Spice up with 1 tsp of the salt & the pepper.
Step #8 Meanwhile, in a medium bowl, the rosemary (if using), mix the eggs, & the remaining salt.
Step #9 Pour the eggs over the potatoes & reduce heat to low.
Step #10 Cook, without stirring, for 1 min.
Step #11 Stir once & cook this until the eggs begin to set, about 3 mins.
Step #12 Transfer to oven & bake this until set, about 8 mins.
Step #13 Use a knife to loosen the omelet from the side of the skillet.
Step #14 Slide or invert it onto a plate.
Step #15 Slice into wedges.
Step #16 Serve hot or at about room temp.
Step #17 Tip: A Spanish omelet is traditionally cooked entirely on the stovetop.
Step #18 To try this, when the eggs begin to set, invert it back into the skillet, slide the omelet onto a plate, & finish cooking over med-heat/flame.
Step #19 Or, if you're feeling adventurous, flip it.