Rinse the rice under cold water. In a large pot, bring 6 cups of water to a boil, add a pinch of salt, and add the rice. Lower the heat, cover, and simmer for about 18 minutes until all the water is absorbed and the rice is cooked through. Fluff with a fork and keep warm.

While the rice is cooking, peel and slice the onions into thin strips. Slice the peppers and tomatoes into bite-sized pieces. Peel and dice the potatoes into small cubes.

Heat 2 tablespoons of oil in a large skillet over medium-high heat. Add the cubed potatoes and cook until they are browned and tender, about 10 minutes. Remove the potatoes and set aside.

In the same skillet, add another tablespoon of oil if needed, and add the sliced onions. Sauté until they become translucent, about 3 minutes.

Add the sliced peppers to the onions in the skillet and cook for another 3-4 minutes until they start to soften.

Add the diced tomatoes to the vegetables, season with salt and pepper, and let cook for an additional 3 minutes. Remove the vegetable mixture from the skillet and set aside.

In the same skillet, add the remaining tablespoon of oil. Increase the heat to high and add the beef, cut into thin strips. Cook the beef until it's browned on all sides, about 5-7 minutes.

Return the cooked potatoes and vegetable mixture to the skillet with the beef. Stir everything together, and check for seasoning, adding more salt and pepper if needed. Cook for another 2-3 minutes to combine the flavors.

Serve the beef and vegetable stir-fry over the cooked rice, ensuring each plate gets an even amount of beef and vegetables. Enjoy your meal!