Cut the beef into 1-inch cubes and place in a zip-top bag on a plate or in a dish.
Mince or paste the garlic.
In a bowl, whisk together the garlic, brown sugar, 2 tablespoons of soy sauce, and the fish sauce. Pour into the bag with the beef and move around to coat. Marinate at room temperature for 30 minutes to 1 hour.
Thinly slice the red onion. In small bowl, combine the juice from 1 lime, granulated sugar, and remaining 1 tablespoon of soy sauce, and pour the mixture over the red onion.
Cut the tomatoes into eighths. Quarter the second lime.
On a platter, arrange the watercress and tomato.
Cut the whites and light green parts of the green onions into 1-inch pieces. Cut the bell pepper into 1-inch pieces as well.
After the beef has marinated for 30 minutes to 1 hour, remove from the marinade, and throw out any excess marinade. Sprinkle the cornstarch over the beef and toss to coat.
Heat a large skillet over high heat and add 1 tablespoon of vegetable oil. When the oil is shimmering, add the green onion and bell pepper and sear for 2 minutes, shaking the pan. Remove from the pan and add to the bed of watercress and tomato.
Top the vegetables with the marinated red onion.
If needed, add another 1 tablespoon of oil to the pan so that it is evenly coated to cook the beef. Heat the oil until shimmering, add the beef and sear on sides, shaking the pan, until it is browned on all sides. This should take 3 to 5 minutes total (about 1 min per side). Note: If your skillet is not big enough to cook the beef in a single layer with some room around each piece, cook it in 2 batches. It's very important not to crowd the pan so that you get a great sear.
Season the beef with the Maggi and give it a final toss before removing it from the pan.
Remove the beef from the pan to the vegetable platter. Garnish with cilantro and lime slices, and serve with rice, if desired.