Start by frying the diced potatoes in 2 Tbsp of olive oil in a large skillet over medium heat. Fry until golden brown and crispy, stirring occasionally to prevent the potatoes from sticking to the bottom of the pan and burning. This usually takes about 20 minutes.
For the sauce, start with a large, deep skillet. Warm 2 Tbsp of olive oil over medium heat, then cook the onion slices until tender (about 10 minutes). Add the garlic and cook for a further minute, stirring to prevent burning.
Add the tomato sauce, water, and all of the remaining seasonings. Bring the sauce to a simmer and allow to thicken, about 15 minutes. Allow it to cool just a bit, then transfer to a blender or food processor and purée until smooth. Return the sauce to the skillet and warm over a low heat, adding any additional seasonings for an extra spicy sauce, if you prefer.
When the sauce is warm and the potatoes are crispy, you're read to serve! We always had it served with the sauce on the side in Spain, but you're welcome to just pour the sauce right over the potatoes if you hate order and structure in your life. Now sit back and enjoy your little taste of Spain!