~2 c. cold leftover potatoes, sliced
1/2 white onion, sliced
3 Tbsp. olive oil
1 Tsp paprika (smoked paprika is extra tasty in this)
1 tsp. salt
1 tsp. pepper
Optional: other leftovers
Heat a large nonstick pan with the olive oil. Add the onions and cook 1-2 minutes. Add the potato slices in a single layer, and cook on medium-low until the edges start to crisp.
Crack the eggs into a large bowl and add the salt, pepper, and paprika. TIP: taste the eggs to see if the salt is right--don't fear the reaper. Dump the hot potatoes into the bowl and stir. I think this step allows the potatoes to get thoroughly mixed throughout the eggs and jump-starts their cooking. It also allows you to add more oil to the pan if you think it's necessary.
Dump the egg mixture back into the pan. Cook for ~10 minutes. Personally, I leave the lid on and allow it to cook thoroughly on one side. Other, more adventurous people, flip the tortilla part-way through. You could also put it under the broiler to cook the top if your pan is oven-safe. If you're desperate enough to be making a tortilla out of leftovers late at night, you probably don't want to try flipping it so keep it simple.
Once cooked to your satisfaction, dump onto a cutting board and slice into wedges.