Ingredients:

• 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 (Korean sweet potato 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