For the marinade, combine the garlic, chillies and lemongrass in a mortar and pestle. Grind together till a paste forms. Add the soy sauce and lime juice, and whisk till combined.
In a non-metallic dish or container, combine the marinade and mackerel fillets. Cover and let marinate for at least 1 hour.
Boil the rice noodles for 8 minutes, or till tender. Drain well, rinse under running cold water and set aside.
Preheat the grill. Remove the mackerel from the marinade, discarding the marinade. Remove any remaining bones from the mackerel fillets. Cut the fillets in half lengthways. Lay on a baking tray lined with aluminium foil.
Grill the fillets, skin side down, for about 4 minutes, till the fish flakes easily with a fork. Remove and let cool.
Fill a shallow bowl or pie dish with tap water. Submerge one spring roll wrapper in water for a few seconds, only till softened enough so that it is malleable. Lay the wrapper on a clean plate or cutting board and start layering the filling to one edge of the wrapper: add a bit of rice noodles, a small strip of lettuce, a couple of slices of cucumber, a few sprigs of coriander, some beansprouts, a piece of mackerel and a sprinkling of spring onion. Roll up halfway, then tuck in the ends and continue rolling to the other edge. Repeat with the remaining wrappers and filling ingredients.