Start by rinsing the long-grain rice under cold water until the water runs clear. This helps to remove excess starch and keeps your rice from becoming gummy. In a rice cooker or pot, combine the rinsed rice with 2 cups of boiling water (minus 2 tablespoons). Bring it to a boil, then reduce to low heat, cover, and let it simmer for about 15-20 minutes until the water is absorbed and the rice is fluffy.
While the rice is cooking, peel and cut the russet potatoes into fries or wedges. Heat about 3 tablespoons of avocado oil in a large skillet over medium-high heat. Once hot, add the potatoes and fry them until golden and crispy, about 8-10 minutes. Make sure to stir occasionally for even cooking. Once done, remove them from the skillet and place them on a paper towel to drain excess oil.
In the same skillet, add the remaining 2 tablespoons of avocado oil. Once heated, add the red onion and sauté for about 2 minutes. Next, add the yellow bell pepper, jalapeño, and minced garlic, cooking for another 2-3 minutes until the vegetables are tender but still crisp.
Slice the beef tenderloin into thin strips against the grain. Push the sautéed vegetables to the side of the skillet and add the beef to the center. Sprinkle with ground cumin if using, and cook for about 3-4 minutes until the beef is browned and cooked to your desired doneness.
Once the beef is cooked, stir in the soy sauce, oyster sauce, distilled white vinegar, and beef stock. Mix everything together and add the Roma tomatoes. Cook for an additional 2-3 minutes until the tomatoes are slightly softened and the flavors meld together.
Finally, fluff the cooked rice with a fork and serve it on plates. Top with the Lomo Saltado mixture, adding the crispy potatoes on the side. Garnish with freshly minced cilantro and optional green onions for a pop of color and flavor. Enjoy your homemade Lomo Saltado!