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 eggs, chicken stock or water, and a generous pinch of salt in a large mixing bowl and whisk until just scrambled. Gently stir in potato mixture. Let stand about 10 minutes.
3. Meanwhile, lend salsa verde ingredients. Don't over process, so as to leave a sauce with some texture.
4. Heat reserved oil in a 8-9" non-stick or well-seasoned cast iron skillet over medium high heat. Pour egg and potato mixture into skillet and flatten potatoes with a spatula until top is fairly even. Reduce heat to medium-low. Cook, running a thin spatula around the edge to let uncooked egg run underneath. Continue cooking until top is slightly wet but not liquid, about 6-8 minutes. Run thin spatula completely under tortilla to ensure it is not sticking to the skillet. Top skillet with a plate slightly larger than the skillet and carefully flip tortilla out onto the plate. Slip tortilla back into skillet top-side down (if the skillet looks a little dry, add a little more oil first. Shake skillet to straighten tortilla, then reduce heat to very low and continue cooking until the center is cooked through. Serve warm or at room temperature with salsa verde.