There's been a lot of rumor about the origins of this dish; I'll let others fight it out and just focus on its deliciousness! I think that the human palate is drawn to the combination of savory, sour, sweet, salty and spicy. I think that's why we all love this dish. There's a huge difference between the popular sambal oelek and Chinese chili garlic sauce. Sambal is very easy to find but it's twice as hot as chili garlic. So, if you are using sambal, use half the amount.
For the chicken: Place your broccoli florets in a medium bowl and add 2 tablespoons water. Cover with plastic wrap and microwave for 2 minutes, until tender. Drain and set aside until ready to use.
Heat the oil in a 4-quart (3.8-liter) Dutch oven to 375 degrees F (190 degrees C) using a frying thermometer. Rinse the chicken in cold water and pat dry with paper towels. Mix 1 1/2 cups (335 grams) of the tempura flour and 1 cup (240 milliliters) water into a thick batter. It should look like thick pancake batter. Dredge the chicken cubes in the 1 cup (223 grams) of tempura flour and then coat them in the batter. Fry them in two batches until golden brown and crispy, 6 to 8 minutes. Drain the chicken on paper towels or a rack.
Heat a wok or large skillet to high and add 2 tablespoons (30 milliliters) of oil. When you see the first wisps of white smoke, stir in the fried chicken, onion, chiles, broccoli, and 1/3 of the ginger and garlic and cook them for about 30 seconds.
For the sauce: While the chicken cooks, add the oyster sauce, hoisin sauce, chili garlic sauce, sugar, white vinegar, soy sauce, cornstarch slurry, red food coloring if using, 1/3 cup water and the remaining chopped ginger and garlic into a 1-quart (946-millileter) saucepan and bring the heat up to medium. Whisk gently as it comes to a simmer. Allow it to simmer and keep whisking it for about 5 minutes, until the sauce thickens. Remove from the heat and reserve it.
Stir in the sauce and allow it to coat the chicken and simmer. Cook it, folding all the ingredients until they are well coated, about 2 minutes. Garnish with the scallions.