Combine the sauce ingredients in a small bowl and whisk until sugar is dissolved. Chop up the vegetables and slice the tofu.
Boil the sweet potato noodles according to the packet instructions. Drain the water out and place it in a large mixing bowl. Pour half of the japchae sauce mixture and mix it well with the noodles.
Stir fry the tofu and mushrooms for 1-2 minutes with 1 tbsp of vegetable oil. Season with salt and pepper to taste. Remove from pan and place it over the noodles.
In the remaining pan, drizzle more oil and saute the onions and 1tbsp minced garlic. Follow with the carrot, bell peppers, spinach and spring onions. Stir fry for 1-2 minutes or until just cooked through. Place them into the mixing bowl.
Pour the remaining sauce, and add 1 tbsp of chopped garlic and toasted sesame seeds. Combine everything well.
Make a taste test and add more soy sauce or sugar if needed. Transfer to a serving plate and garnish with toasted sesame seeds! Enjoy!