Begin by placing the beef into a bowl and marinate it generously with soy sauce, freshly squeezed lemon juice, and a good grind of black pepper. Allow this to rest for at least an hour — though for truly tender and flavorful results, marinating overnight is ideal to let the flavors deeply seep into the meat. Next, heat olive oil in a pan over medium heat and add half of the sliced onions. Sauté them until they soften and become translucent, gently releasing their sweetness without burning. Once soft, transfer these caramelized onions onto a plate and set aside. Drain the beef from its marinade, reserving the liquid for later use. Using the same pan, add a little more oil if necessary and quickly pan-fry the beef slices for about one minute on each side. This quick sear locks in the juices and builds flavor. Remove the beef and set aside. In the same pan, add the minced garlic and the remaining raw onions. Sauté until the onions soften and the garlic becomes fragrant, being careful not to let them brown too much. Pour in the reserved marinade along with a cup of water. Bring this savory sauce to a boil, allowing it to meld the flavors together. Return the beef to the pan, cover it, and reduce to a simmer. Let it cook gently until the meat turns tender, adding more water as needed to maintain a saucy consistency. Season the dish with additional salt and freshly ground black pepper to taste. For the finishing touch, scatter the pan-fried onions on top before transferring the bistek to a serving platter. Serve your hot, aromatic Bistek with steamed rice to soak up the luscious sauce. Enjoy the harmonious blend of savory soy, zesty lemon, tender beef, and sweet onions in every mouthful!