Heat 2 teaspoons oil in a medium non-stick skillet over medium heat. Add onion and cook, stirring, until translucent, 3 to 4 minutes. Add potatoes, thyme and paprika and cook for 2 minutes more.
Lightly whisk eggs and egg whites in a large bowl. Gently stir the potato mixture into the eggs along with spinach, salt and pepper until combined. Wipe the pan clean; add the remaining 1 teaspoon oil and heat over medium heat. Pour in the egg mixture, cover and cook until the edges are set and the bottom is browned, 4 to 5 minutes (it will still be moist in the center).
To flip the tortilla, run a spatula gently around the edges to loosen them. Invert a large plate over the pan and turn out the tortilla onto it. Slide the tortilla back into the pan and continue cooking until completely set in the middle, 3 to 6 minutes. Serve warm or cold.
Note: you can add 1/2 cup shredded Manchego or Jack Cheese if desired to the egg mix in step 2.
There are 398 calories in 1 serving of Spanish Tortilla.