Prepare all the ingredients, mise en place style – this dish cooks quickly!
Heat a wok or large sauté pan over high heat until hot. Pour in the oil to lightly coat the bottom of the pan and heat the oil for 2 to 3 minutes, until very hot.
Toss in the beef and quickly sear both sides for a few seconds each until it begins to brown, about 30 seconds total.
Add the onion and shake the pan or use tongs to flip them a few times, then add the tomatoes right away. Fry the saltado until the edges of the onions color in a few spots and the tomatoes barely begin to soften about 30 seconds.
Add the saltado and soy sauces on top of the stir-fry ingredients.
Scatter the scallions and cilantro on top of the stir-fry and toss everything together one more time.
Spoon the lomo saltado straight out of the pan alongside the roasted fingerling potatoes.