It may seem an unlikely combination, however, the saltiness and crispness of the prosciutto works well with the rockling. The prosciutto which becomes crispy during the cooking protects the fish and prevents it from drying out. Melted herb butter makes an easy sauce. The butter is melted in the hot pan, after the fish is removed and poured over the plated fish, you only need a little.
I keep little pats of basil butter in the freezer, it is made in Summer when my basil is in season. You can get a recipe or read about making basil butter here. Alternatively, a small amount of herb butter can be made by mixing some softened butter or a mixture of olive oil and butter with freshly chopped herbs (basil, thyme, oregano) and a little crushed garlic.
Remove fish from pan, add the basil/herb butter and allow to melt. Pour over fish.