Slice tofu in half then into 1/4-inch-thick pieces and pat dry to remove excess moisture. Lightly season with salt and pepper and set aside.
Combine the sauce ingredients in a bowl. Add garlic and mix to combine.
Heat oil in a large skillet over medium heat. Carefully add the tofu pieces and fry both sides until golden brown, for about 1-2 minutes on each side.
Pour the sauce mixture over the tofu. Reduce the heat to low, cover, and cook for about 5 minutes. Remove the cover and cook for about 2-3 minutes until the sauce is reduced.
Finish with sesame oil and serve with japchae!
For the Noodles
Bring pot of water to a boil. Add noodles and cook until soft and chewy but not mushy, stirring occasionally, 5 to 6 minutes. Drain.
In a large bowl, toss noodles with sesame oil, honey, and soy sauce.
For the Japchae
In a medium bowl, whisk together soy sauce, sesame oil, gochujang, garlic, and black pepper.
Return skillet to medium-high heat and heat oil. Add red onion, carrot, mushroom, spinach, and salt. Cook until just beginning to soften, about 4 minutes. Let cool slightly, then transfer to bowl with gochujang mixture. Toss to combine.
Once noodles are cool, add in all vegetables, tossing gently to combine.