Use any fish with a firm texture and mild, sweet flavor. White fish such as halibut, sea bass or cod.
Preheat oven to 350º.
Wrap 2 or so slices of pancetta around each piece of fish, making sure to cover end to end.
Place fillets in a very hot skillet lined with olive oil. Cook for about 2 minutes on each side, making sure all sides are browned. Carefully remove fillets from pan and transfer to an oven-proof dish. Place in oven for about 5 minutes to cook through. Remove from oven and let fish rest for another 5 minutes before serving.
Grain Mustard Sauce:.
In a medium-sized non-reactive pan, combine the clam juice, white wine, vinegar, shallot, and bay leaf. Place over high heat and reduce to ¼ cup of liquid (about 15 to 20 minutes). Add cream and return to a boil. Continue to boil for about 2 minutes.
Remove from heat and slowly whisk in butter. Whisk in mustard and season with salt and pepper if desired. If not using right away, keep the sauce warm in a double boiler until needed, adding a little hot water if the consistency becomes too thick.
Place a fillet in the center of each plate and drizzle with mustard sauce. Garnish with a light salad or seasonal vegetable if desired.