In a measuring cup, mix together the water, oyster sauce, lime juice, cornstarch, salt, and red pepper flakes. Mix well until the cornstarch is dissolved.
In a large pan or wok, heat some oil on high. If your steak slices are really long, cut them down into about 3" pieces. Pat the pieces of steak dry with a paper towel. When the pan is hot, sear the meat in 2 or 3 batches, so that you don’t overcrowd the pan and steam the meat. Sprinkle each batch lightly with salt and pepper and cook for about 2 minutes total, or just until browned and almost cooked through. Transfer to a plate.
Add more oil to the pan, along with the scallion whites and garlic. Saute together, for about 1 minute. Whisk the sauce one more time, and add it to the pan. Cook the sauce, while scraping the bottom of the pan with a wooden spoon or spatula, to pick up the brown bits. When the sauce comes to a bubble, add back the steak, along with the peppers and scallion greens. Toss and cook for 1 minute more. Remove from the heat and serve with rice.