Heat oil (and butter if using) in a deep pot on medium heat. Add cumin and optional bay leaf. When cumin sizzles, add onion and cook until edges turn light pink, about 4 minutes.
Add ginger garlic paste and green chilies. Cook 1 to 2 minutes until fragrant. If using tomato, add with a pinch of salt and cook until slightly soft.
Add water or stock. Crumble in stock cube if using. Add turmeric, black pepper, and optional soy. Simmer gently 8 to 10 minutes.
Add carrot and beans; simmer 4 to 5 minutes. Add cabbage, capsicum, and mushrooms if using; simmer 4 to 5 minutes until vegetables are tender. Adjust salt; add sugar if needed.
Noodles and serve
Either cook noodles directly in the simmering broth for the shortest time on the packet (stirring, adding hot water if too thick), or boil noodles separately until almost al dente and ladle soup over bowls.
Turn off heat. Stir in cilantro and spring onion. Serve with lemon wedges.
Notes
For leftovers, store broth and noodles separately to avoid mushy noodles.Gluten-free: rice noodles plus omit wheat-based soy or use certified gluten-free soy sauce.Optional richness: whisk 1 teaspoon peanut butter into a little hot broth off heat for a milder, rounder kid-friendly bowl.Spice: add extra slit chilies or chili oil at the table without changing the base broth.