First, make the marinade. Place the garlic, chillies and lemongrass in a mortar and pestle and grind into a paste (you can also use a small food processor). Add the soy sauce and lime juice and whisk until combined.
In a non-metallic bowl, pour the marinade over the mackerel fillets. Cover and let marinate at least 1 hour.
Boil the noodles until tender, about 8 minutes. Drain, rinse with cold water and set aside.
Preheat the grill. Remove the mackerel fillets from the marinade and cut in half lengthwise. Remove any remaining bones. Place on a foil-lined baking tray, skin side down. Grill for 4 minutes, until the fish flakes easily with a fork. Remove and let cool. Discard the marinade.
Make the spring rolls: working one at a time, soak a spring roll wrapper in cold water for a few seconds until just malleable. Lay it on a cutting board and add filling to the centre: some rice noodles, a strip of lettuce, a couple of cucumber slices, a few sprigs coriander, some beansprouts, a piece of mackerel and a sprinkling of spring onion. Roll up the spring roll halfway, tuck in the ends, and continue rolling. Repeat until the filling is used up.
Cut the spring rolls in half just before serving with Vietnamese dipping sauce.