Slice the steak into 1/2 cm strips across the grain, using partially frozen meat if it helps you cut cleanly, then season with salt and black pepper. The strips should look thin and even, with the long muscle fibers running across each piece rather than along it.

Heat the oil in a large wok or frying pan until it is very hot, then sear the beef for 4 minutes without moving it before turning and cooking for 1 to 2 minutes more; work in 2 batches if needed. The beef should sizzle loudly on contact and develop browned edges while the pan stays dry rather than filling with juices.

Transfer the beef aside and cook the ginger, garlic, and red pepper flakes in the same pan for 1 minute. Stir constantly until the aromatics smell warm and fragrant, stopping before the garlic darkens.

Pour in the tamari, water, and brown erythritol, bring the mixture to a boil, then lower the heat to medium and simmer for 3 to 4 minutes. The sauce is ready when bubbling becomes slightly slower and the liquid looks glossy enough to coat a spoon.

Return the beef to the skillet with the sliced red bell pepper and cook for 2 to 3 minutes. The pepper should soften around the edges while keeping a firm bite, and the sauce should cling to the beef.

Turn off the heat, stir through the spring onions, then finish with toasted sesame seeds, more black pepper, and a drizzle of toasted sesame oil if desired. The scallions should stay bright and fresh, with their aroma lifting the rich glaze.