Peel and finely chop the onion.: The moment you begin chopping onion , a sharp, sulfurous scent will fill the air, and as you cut them finely they will release more of their sweet juices during cooking. Finely chopped pieces soften more quickly and meld into the broth, creating a silky base. A common mistake is chopping too roughly, which leads to larger, undercooked bits that distract from the soup's smooth texture.
Peel and mince/grate the garlic and ginger.: When you mince garlic and grate ginger , the kitchen fills with a warm and slightly pungent aroma that signals flavor is building. Minced garlic melds into the onions while grated ginger adds a bright, citrusy warmth. Avoid adding them raw to the hot soup without sweating first, because raw garlic can taste harsh and raw ginger can be overly fibrous.
Finely chop the red chilli (if you want to reduce the heat remove the seeds and membrane).: Chopping the red chilli releases its fruity heat and oils which scent the air and flavor the dish. If you prefer milder heat, remove seeds and membranes; leaving them in will deepen the warmth with each spoonful. Take care not to touch your face after handling chillies, and wash hands thoroughly to prevent accidental eye irritation.
Heat the oil in a large pan over a low heat.: As the oil warms gently, it will shimmer faintly and become the medium that helps extract flavors from the aromatics. Using low heat prevents the oil from smoking and keeps the aromatics from browning too fast. If the oil smokes, lower the heat and start again, because burnt oil will create a bitter background note.
Add the onion, garlic, ginger, chilli and salt. Then fry for 3-4 minutes until everything has softened but not coloured.: During this gentle frying phase, the aromatics will become soft and fragrant, and you will notice a sweet, mellow scent replace the earlier sharpness. The salt draws moisture out of the onions, accelerating their softening. Avoid frying at high heat, which will brown the aromatics and shift the flavor away from the bright, fresh base we want.
Add the turmeric and vegetable stock and bring to a simmer.: When you add the turmeric the oil picks up a golden color and an earthy aroma, which the vegetable stock carries throughout the pot as it comes up to a gentle simmer. A simmer, not a rolling boil, is key here, as violent bubbling can break down delicate flavors. If the stock tastes flat, check your seasoning, because the simmer will concentrate and reveal imbalances.
Add the potatoes and simmer for 15 minutes.: As the potatoes simmer, the broth will take on a fuller body and the potatoes will become tender, releasing some starch to slightly thicken the soup. Listen for a steady, gentle bubbling and watch for the potatoes to pierce easily with a fork. Do not over simmer beyond tenderness unless you want a thicker, more mashed texture, because falling apart spuds change the soup's mouthfeel significantly.
Cut the baby sweetcorn into 3 pieces and add them into the soup. Cook for a further 5 minutes. (See NOTES): Adding the baby sweetcorn at this point gives it time to warm through and soften slightly while retaining some bite. You will notice a faint corn sweetness in the steam and a pleasing pop in texture when you bite into the kernels. If using canned corn, follow the note and wait to add it later to avoid overcooking and a mushy result.
Add in the coconut milk and the frozen peas. Bring back to a simmer and cook for 5 minutes.: Stirring in the coconut milk will create a silkier, creamier broth and mellow the spices, while the peas will turn bright green as they heat. As it simmers, the soup should look glossy and slightly thicker, and the aroma will shift toward cushiony coconut with herbaceous highlights. If the coconut milk splits, lower the heat and stir gently; high heat can separate the fats.
Serve garnished with fresh cilantro, more chopped chilli and some wedges of lime.: At the finish, the smell of fresh cilantro and the sharpness of lime juice will lift the rich broth, and extra chopped red chilli provides a visual pop and optional heat. Garnish just before serving to preserve the herb's bright aroma. Over-garnishing early will wilt the herbs and mute their flavor, so add them at the last moment.