Super simple prep -- just sauté onion and potatoes in olive oil until the potatoes are soft.
Then remove them from the heat into a bowl, but leave the oil in the bottom of the pan. Let the potato/onion mixture cool for several minutes until it isn't in danger of cooking the egg. In a separate bowl, beat the eggs together, and then mix in with the potatoes. Pour all of it into the pan.
You need to continually move the pan to make sure that the egg doesn't stick. Once it is cooked on the bottom, turn it over (either by inverting onto a plate, or just with big spatulas) and continue cooking.
We ate this for breakfast with a little hot sauce on the side. It doesn't seem flavorful enough for a lunch or dinner, but was great as a first meal.
onion -- $1.01
potatoes -- $1.10
Total Cost of Spanish Tortilla with Potatoes and Onion: $2.11