In a large skillet, add the vegetable oil and heat over medium-high heat.
Cook the sliced potatoes in the skillet, spreading them out evenly. Cook for about 5 minutes or until they are cooked through and golden brown.
Remove the cooked potatoes from the skillet and place them on paper towels to drain excess oil. Season with salt and pepper to taste.
In the same skillet, carefully remove most of the oil, leaving about two tablespoons behind.
Over medium heat, cook the sliced steak and chopped garlic until the meat is almost fully cooked, about 4 minutes.
Add the thinly sliced onions and peppers to the skillet and cook until the onions start to soften, approximately 2 minutes. If the mixture starts to dry out, add about 1 cup of water.
Add the diced tomatoes, tomato sauce, kosher salt, and ground black pepper to the skillet. Cook for an additional 2 minutes.
Once the tomatoes are soft, add the cooked potatoes back to the skillet and mix well with the other ingredients. Cover the skillet with a lid and simmer on low heat for about 5 minutes to allow the flavors to meld together.
Taste the dish and adjust the seasoning with additional salt and pepper if needed.
Serve the Ranchero Steak with Potatoes hot, garnished with fresh cilantro or diced avocado if desired.