Trim the ends from the spring onions/scallions then cut into lengths, roughly 2 - 3 inches (5 - 7 ½cm) in length, both the white and green parts. Cut thicker parts in half length-wise as well. Remove the thicker stems from the dill then cut the top part into lengths, also around 2 - 3 inches (5 - 7 ½cm).
Prepare the rice vermicelli noodles for serving according to packet instructions - typically you simply soak in boiling water to hydrate and soften, then drain and set aside (ideally drain as the fish cooks to be still warm). Combine the nuoc cham ingredients, if using, in a small dish.
Warm the oil in a small-medium skillet/frying pan over a medium heat - add a little more oil if needed so you have a thin layer all over. Add the pieces of fish and fry for a few minutes either side until a lightly golden crust forms on each side and the fish is almost cooked through.
Once the fish is seared on all sides and largely cooked, push the fish to the sides of the skillet/pan and if needed, add a little more oil in the middle. Add the spring onions/scallions and dill in the middle and wilt them down, turning over as needed so all pieces can wilt in the heat. It should only take a minute or two.
Remove the fish and greens from the heat and serve over the rice noodles, topped with some sauce and peanuts. You may want to serve just some of the fish and greens at a time over the noodles, then take more from the shared skillet as needed (or divide the fish and greens onto separate individual dishes). To eat, pick up some of each of the ingredients then dip in sauce before eating.