Heat a splash of olive oil in a nonstick pan.
When hot, add the potatoes and cook until crisp and browned in most places.
Drain on paper towels.
Put the beef in the same pan and add a teaspoon each of salt, garlic powder and cumin, and ½ teaspoon of black pepper.
Cook for 8 to 10 minutes, or until browned.
Add the garlic and onion and cook for another few minutes.
Add the tomatoes, water, serrano pepper, a pinch of oregano, and some salt to a blender and puree until smooth.
Pour the pureed mixture in with the beef.
Bring it to a boil and add more salt if needed.
Stir in the potatoes, adjust the seasonings as needed
Simmer for another 7 to 10 minutes, or until thickened.
Serve hot.