In a medium bowl, combine soy sauce, sugar, minced garlic, and black pepper. Add thinly sliced beef short ribs and marinate for 15-20 minutes.

Boil water and cook sweet potato noodles (dangmyeon) for 6-8 minutes or until tender. Drain and rinse under cold water.

Whisk soy sauce, sugar, minced garlic, sesame oil, salt, and pepper in a small bowl to create the dressing.

Heat vegetable oil in a skillet, add onions and shiitake mushrooms, and stir-fry for 4-5 minutes until translucent.

Add carrots and bell peppers to the skillet and stir-fry for 3-4 minutes, then add baby spinach and cook until wilted.

In the hot skillet, cook the marinated beef for 5-7 minutes until browned, then transfer to the mixing bowl.

Combine noodles, sautéed vegetables, and beef in the mixing bowl with the dressing and toss gently to mix.

Plate the Japchae and garnish with toasted sesame seeds. Serve warm or as a salad.