Begin by gathering and preparing all your ingredients. Slice the beef into thin strips, then chop the carrots and onion into matchsticks. Rinse the spinach and slice the green onions. Measure out your soy sauce, sesame oil, and brown sugar in separate bowls for easy access.

In a large pot, bring water to a rolling boil and add a splash of oil. Add the sweet potato noodles and cook for 5-6 minutes until they become soft yet chewy. Drain and set aside.

Heat a large skillet over medium-high heat and add olive oil. Once hot, add the beef, searing without stirring for 3-4 minutes until golden brown. Season with soy sauce, a splash of sesame oil, and brown sugar. Cook until fragrant, then transfer to a plate.

In the same skillet, add more oil if necessary, then toss in the carrots and sliced onion. Stir-fry for 3-5 minutes until softened. Add remaining soy sauce, sesame oil, brown sugar, and minced garlic, cooking for an additional minute.

Reduce heat to medium, add the cooked noodles to the skillet along with the sautéed vegetables. Return the beef and gently toss to combine, then fold in the spinach until it wilts slightly.

Transfer japchae to a serving dish. Enjoy warm or at room temperature.