In a small bowl, combine all the sauce ingredients until well combined. Set aside.
Heat a large frying pan with oil on medium-high heat. Separate the thinly sliced beef tongue and add them to the frying pan. Cook until the tongue is brown and no longer pink.
Add the garlic and saute until the garlic is fragrant. Then, add the sauce and cook until the sauce is reduced.
Add the finely chopped bird's eye chilis and scallions. Stir until just combined. Turn off the heat.
Serve the beef tongue slices on top of serving bowls. Garnish with more chilis and scallions if desired.
Notes
I really recommend adding chilis to give a kick to the gyutandon. If you want it less spicy, you can reduce the number of chilis or remove the chili seeds.