For the beef
6?ounces?tender?beef?(strip, rib eye, or?sirloin), sliced against the grain into 1/4-inch thick strips
2?teaspoons?soy sauce
1?teaspoon?rice wine vinegar
1?teaspoon?sugar
1?garlic clove, grated
For the japchae sauce
6?tablespoons?soy sauce
3?tablespoons?sugar
1?tablespoon?toasted sesame oil
To make the japchae
12?ounces?dangmyeon noodles ()?
2?large?eggs
1/4 cup vegetable or canola oil, divided, or as needed
1/2?medium?yellow onion, cut into 1/2-inch slices
1?large?carrot, cut into 3x1/4-inch matchsticks
3?ounces?shiitake mushrooms, stemmed and cut into 1/4-inch slices
1 small (about?3?ounces)?red bell pepper, cored and cut into 1/2-inch slices
6?green onions, trimmed and cut into 2-inch pieces, whites and greens separated
2 cups?baby spinach?(2?ounces)
Kosher salt?and freshly ground black pepper, to taste
Toasted sesame seeds, for serving