Fill a large pot with cold water and the diced potatoes. Bring to a simmer over medium heat, and let cook for about 6 minutes or until the potatoes are tender, but not mushy.
While the potatoes are cooking, heat a large sauté pan to medium-low heat and add the onion and serrano peppers. Cook the onion and peppers for 3-4 minutes or until the onion is tender and translucent. If the onion begins to stick add some water or vegetable stock to the pan.
Add the diced tomatoes to the pan and cook for about 6 minutes, or until the tomatoes begin to break down and become more of a sauce.
Drain the potatoes, but reserve 1/4 cup of the potato water.
Add the potatoes and the potato water to the pan. Mix well and cook for 1 more minute.
Season with salt and pepper and serve with warm corn tortillas and salsa.