Rub 1 tablespoon salt onto beef tongue; place on a plate. Let sit until flavors combine, about 5 minutes.
Bring a pot of water to a boil. Dip beef tongue into boiling water with a slotted spoon for 30 seconds. Remove, draining excess water; transfer to a plate.
Heat sesame oil in a skillet over medium heat. Add garlic; cook and stir until fragrant, 1 to 2 minutes. Stir in onions; cook and stir until soft, 2 to 3 minutes. Add beef tongue and more sesame oil if needed; cook and stir until tongue is browned, about 3 minutes.
Stir sake, dashi powder, salt, and pepper into the skillet; simmer until flavors combine, 2 to 3 minutes.