Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Slice the russet potato into thin wedges, toss with a pinch of sea salt and black pepper, and roast for 25 minutes until golden and crisp.
Slice the beef sirloin into thin strips and season with the remaining sea salt and black pepper.
Heat the avocado oil in a large skillet or wok over high heat until shimmering.
Add the beef strips in a single layer and sear for 2 minutes until browned and caramelized, then remove from the pan and set aside.
In the same pan, add the sliced red onion and minced garlic, sautéing for 2 minutes until the onion is slightly softened but still has a bite.
Add the sliced roma tomato, tamari, and red wine vinegar, stirring quickly to deglaze the pan and create a light sauce.
Return the beef and the roasted potato wedges to the skillet, tossing everything together for 1 minute to coat.
Garnish with chopped fresh cilantro and serve immediately while steaming hot.