Combine all of the ingredients for the marinade in a sauce pan. Heat over medium low heat. Cook and stir occasionally, until the sugar is completely dissolved. Remove the pan from the stove to cool off.
Cut the pork along the grain, into 2 strips about 2 inches wide and 1 inch thick. Transfer the pork to a gallon-sized ziplock bag. Pour half of the marinade onto the pork and save the rest in an airtight container in the fridge (for later use). Seal the bag and press as much as air out as possible. Rub the bag so that the pork is covered well with the marinade. Let marinate at room temperature for 3 hours, or in the fridge overnight.
Preheat oven to 400 degrees F. After the oven is preheated, turn on broiler. Place oven rack in the lower third of the oven, about 10 inches from the broiler element. Add the red food coloring into the remaining marinade. Line a baking pan with aluminum foil and add 1/4-inch of water. Place a baking rack on top. Drain pork loin and discard the marinating liquid. Transfer pork onto baking rack.
Bake for 20 to 25 minutes in total, until the internal temperature registers 140 to 150 degrees F (60 to 70 C). Flip pork every 4 to 5 minutes, 3 times, until the surface is cooked. In the last 5 to 6 minutes, flip the pork every 1 to 2 minutes, and generously brush marinade onto the pork using the remaining marinade we saved earlier. When it’s finishing up, the pork should be covered with a thick coat of marinade, slightly charred/caramelized, with the inside is still a bit pink (or just cooked through).
Remove the pan from the oven. Tent loosely with aluminum foil. Let rest for 10 minutes. Slice and serve warm or cold. You can serve the char siu by itself over steamed rice, atop noodles, or in other dishes.