If you like the taste of french fries, you’ll love the Spanish tortilla. When made correctly, it is a delicious half-inch thick “cake” of fried potatoes mixed with fried eggs and onions. After cooking, the tortilla can be cut into pizza-like triangles to serve 4-6 people, or cut into squares to give a whole group a bite-sized toothpick sample.

1 cup olive oil
four large potatoes (peel and dice 2mm thick)
salt to taste
one large onion, thinly sliced
four large eggs.

Some people add thin slices of red pepper together with the onion. Heat the oil in a 9-inch skillet, add potato pieces, one slice at a time so that they don’t stick. Alternate layers of potato and onion. Cook slowly, medium flame. Do not fry. Turn occasionally until potatoes are tender, but not brown. They must be loose, not “in a cake”. Beat eggs in a large bowl with a fork. Salt to taste. Drain potatoes. Add potatoes to beaten eggs, pressing them so that eggs cover them completely. Let sit for 15 minutes. Heat 2 tbsps of the oil in large skillet. Add potato-egg mixture, spreading quickly. Lower the heat to medium-high. Shake pan to prevent sticking (crucial step!!) When potatoes start to brown, put a plate on top skillet and flip to cook other side, adding another tbsp of oil. Brown on the other side. Can flip three or four times for better cooking.
Source: http://www.xmission.com/~dderhak/recipe/tortilla.htm