Cut potatoes and tofu into bite-sized pieces.
Chop the onion and green onion into small pieces.
Add cooking oil to a pan and stir-fry the potatoes.
When the potatoes are half-cooked, add onion and tofu and continue stir-frying.
Add drained canned tuna and stir-fry together.
Add soy sauce, gochugaru (Korean chili flakes), minced garlic, oligosaccharide, and water, then bring to a boil.
Simmer over medium-low heat until the sauce is thickened.
Add green onion, mix one more time, and transfer to a plate.