In a medium bowl, whisk to combine 1 tablespoon water with baking soda. Add the ground beef and mix to combine. Let sit for 15 minutes.
Meanwhile, in a small bowl, whisk the remaining 1 tablespoon water with cornstarch; set aside. In a separate small bowl, combine stock, black pepper, salt, red pepper flakes, and MSG, if using. Mix well and set aside.
In a large sauté pan or stainless-steel skillet, heat 1 tablespoon oil over high heat until shimmering. Add beef and grated onion and cook, stirring frequently and breaking the beef into small pieces with a wooden spoon, until most of the beef is no longer pink and onions are softened, about 5 minutes. Add minced garlic and cook until fragrant, about 1 minute. Pour in the stock mixture and cook, stirring occasionally, until most of the liquid has evaporated, about 5 minutes. Add the cornstarch mixture and cook, stirring constantly, until the sauce has thickened and coats the ground beef, about 1 minute.
Separate the buns and place a slice of American cheese on each bottom half. Using a slotted spoon, top cheese with about ¼ of the loose meat filling (about 4 ounces per sandwich) and press gently to compact. Top with 2 to 3 dill pickle chips and mustard. Place the top buns on, squish to compact the layers, and serve immediately.
