1. Toss potato slices with salt. Heat oil in a large, heavy skillet with high sides until hot-about 3 minutes. Reduce heat to medium-low and add potatoes in even layers. Cook potatoes, stirring and turning occasionally until the potatoes are about half-cooked. Stir in the onion, reduce the heat to low, and cook until the potatoes are soft, about 15 minutes more. Using a slotted spoon, transfer the potatoes and onions to a colander and let them drain thoroughly. Set aside 2 tablespoons of the oil and discard the rest.
2. Place the eggs, chicken stock or water, and a generous pinch of salt in a large mixing bowl and whisk until just scrambled. Gently stir in the potato mixture. Let stand for about 10 minutes.
3. Heat the reserved oil in a 8-9" non-stick or well-seasoned cast iron skillet over medium high heat. Pour the egg and potato mixture into the skillet and flatten the potatoes with a spatula until the top is fairly even. Reduce the heat to medium-low. Cook, running a thin spatula around the edge to let uncooked egg run underneath. Continue cooking until the top is slightly wet but not liquid, about 6-8 minutes. Run the thin spatula completely under the tortilla to ensure it is not sticking to the skillet. Top the skillet with a plate slightly larger than the skillet and carefully flip the tortilla out onto the plate. Slip the tortilla back into the skillet top-side down (if the skillet looks a little dry, add a little more oil first. Shake the skillet to straighten the tortilla, then reduce the heat to very low and continue cooking until the center is cooked through. Serve warm or at room temperature with salsa verde.Blend salsa verde ingredients. Don't over process, so as to leave a sauce with some texture.