Fry the beef for 5 minutes.

Add the potato, cut into cubes, and continue frying for 5 more minutes.

Blend the tomatoes, onion, garlic, black pepper, cumin, and salt to taste. Add this mixture to the pan along with the whole serrano peppers (these add flavor without making it spicy; for more heat, blend one of the peppers with the other ingredients). Let it simmer until the potatoes are tender, then adjust the salt as needed.