Melt butter or margarine in a large skillet over medium heat.

Add sliced potatoes in batches, cooking until lightly browned and nearly tender on both sides. Season with salt and pepper and remove from the skillet.

Sauté chopped onion in the same skillet until golden brown.

Remove onion with a slotted spoon and add lemon juice to it.

Return the potatoes to the skillet.

Arrange tomato slices over the potatoes, then spread the onion mixture evenly over the tomatoes.

Cover the skillet and cook over low heat for 10 minutes, or until the potatoes are tender.

In a separate bowl, whisk together eggs, cinnamon, and salt.

Pour the egg mixture over the vegetables in the skillet.

Cover and cook over low heat until the eggs are set and the omelette is cooked through.