For the Steak:

1. In a large bowl, combine the sliced steak with soy sauce, lemon or lime juice, minced garlic, sugar, and pepper. Cover and refrigerate for a period of 1 to 24 hours.
2. When ready to cook, take the marinated steak from the refrigerator and place it in a strainer set over the marinade bowl to catch any excess liquid.
3. Heat a large carbon steel or cast-iron skillet over medium-high heat, adding enough oil to coat the bottom. Once the oil is hot and begins to smoke, add the onion slices and cook them undisturbed for 2 minutes until they are browned on one side. Remove them and set aside to maintain their crunch.
4. In the same skillet, add enough marinated steak slices to cover the bottom without overlapping. Sear for 20 to 30 seconds on each side until browned around the edges. Transfer to a plate and continue cooking in batches, adding more oil as necessary.

For the Sauce:

1. Add 1 to 2 teaspoons of oil to the skillet, then stir in the garlic and cook over low heat until fragrant, about 30 to 45 seconds.
2. Incorporate the remaining drained marinade, soy sauce, lemon juice, sugar, beef stock, and pepper into the pan. Bring to a boil over high heat for 2 to 3 minutes, skimming any impurities that rise to the surface if desired.
3. While the sauce is boiling, prepare your slurry by mixing 3 tablespoons of cornstarch or potato starch with 3 tablespoons of water.
4. Add 1 tablespoon of the slurry to the sauce and stir until it thickens.
5. Gently fold in the cooked onions and seared steak.
6. Serve the bistek over cooked rice, garnishing with the onions on top.