Boil 10 cups of water in a wide bowl or pot. Add 1 tsp of cooking oil and some salt. When the water comes to a rolling boil, add the hakka noodles (break them into two halves if needed). Let them cook uncovered for 10 minutes.
In the meantime, finely chop the garlic and spring onions, shred the cabbage, and thinly slice the onion, capsicum and carrot into juliennes.
Toss the noodles once or twice in between so they cook evenly without clumping. Check after 10 minutes—take a small noodle strand and press it; it should be soft yet break with a slight bite (al dente). Immediately drain the excess hot water using a colander and rinse the cooked noodles under cold tap water to stop the cooking process and rinse away surface starch. This keeps the noodles firm and separate.
Heat 2 tbsp oil in a wide kadai or wok on high flame. Saute the finely chopped garlic and half of the spring onions for a few seconds. Add the sliced onion, carrot strips, shredded cabbage and capsicum. Sprinkle sugar and required salt. Keep tossing the vegetables on high flame until 3/4th cooked so they retain their crisp crunch.
Lower the flame and add tomato sauce, green or red chilli sauce, soy sauce and vinegar. Mix quickly on low heat so the sauces blend without burning.
Add the cooked and drained noodles along with black pepper powder. Toss gently on high flame so the noodles are evenly coated with the savory sauces and stir-fried vegetables.
Toss the noodles on high flame for another 1 to 2 minutes. Garnish with the remaining spring onion greens, remove from heat, and serve hot. Enjoy!