In a large pan of salted water cook your pasta following the packet instructions. It should cook for no more than 10 minutes to become al dente (ie with a little bite).
Meanwhile, place the Dutch Yellowtail fillet on a clean surface and season with salt and pepper on both sides. (See the FAQ for notes on deboning, especially if serving to children.)
Lightly score the skin – the will prevent the fish from curling during cooking.
Melt the butter in a large nonstick frying pan over a medium heat.
When the butter is gently bubbling, place the fish fillet in the pan, skin side down.
Cook until the skin is starting to go crisp, about 4 minutes.
Carefully turn the fish using a slotted spatula and cook for a further 3 minutes while basting the top with the butter. Occasionally move the fish gently in the pan to stop it from sticking.
Once the fish is cooked, remove from the pan and set on a plate to rest on a serving dish. (See the FAQ for notes on deboning, especially if serving to children.)
To finish the pasta, add the garlic to the buttery pan and fry for 30 seconds.
Drain the pasta, saving a tablespoon or two of the cooking water. Add the pasta to the pan along with the rosemary and half the parsley..
Toss the pasta in the frying pan, adding the reserved cooking water if you want a little more sauce.
Transfer the pasta into a large serving bowl.
Serve everything to the table with lemon wedges and a scattering of the remaining parsley.
Frying your fillet skin side down in butter over a medium heat should help you get a crisp skin. However, if you find your fillet skin a little less crisp that you'd llike, you can place the fillet on a baking sheet skin side up and under a hot grill for just 30 second or so. This should crisp the skin up nicely.