Bring a medium pot of water to a boil. Add the shrimp, squid, and mussels, and cook for 2-3 minutes, or until just tender. Drain and set aside to cool slightly.

In a large mixing bowl, combine lime juice, fish sauce, and sugar. Stir well until the sugar fully dissolves, forming a balanced dressing. Taste and adjust for sweetness or acidity as desired.

Add the chopped Thai chilies to the dressing. For a more intense spice, lightly press the chilies to release their oils. Stir to evenly incorporate the flavours.

Once the seafood is cool enough to handle, add it to the dressing in the bowl. Toss gently to ensure the seafood absorbs the dressing flavors evenly.

Add the sliced onion, cucumber, and tomato to the bowl. Gently mix to distribute the vegetables with the seafood, ensuring they are well-coated in the dressing.

Fold in the fresh cilantro and any additional herbs, if desired. Mixing gently will prevent bruising the herbs, keeping their flavors vibrant.

Taste the salad, adjusting lime juice, fish sauce, or chili if necessary. This salad should be balanced in sour, spicy, and salty notes, so feel free to adjust these to suit your preference.