Prep vegetables and tofu: Dice onion, slice garlic, julienne carrot, shred cabbage, dice red pepper, chop choi sum, slice spring onions, and cube tofu.
Fry base: Heat oil in a saucepan. Fry onion until golden, add tofu and crisp on all sides. Season. Add carrot, cabbage, and red pepper, then cook until softened.
Make broth: Pour in dashi, add curry roux blocks, and stir until dissolved and thickened into a smooth broth. Season to taste.
Cook noodles: Add fresh udon straight to the broth and stir until separated. For dried udon, boil 2 minutes less than packet instructions, rinse, then add to broth to finish cooking.
Serve: Divide noodles and broth into bowls. Top with choi sum and sliced spring onions. Serve hot.