4 skinless, boneless halibut fillets (about 1pound total weight)
Heat 1½teaspoons oil in a saucepan and add the mushrooms. Cook, stirring, until the mushrooms give up their liquid. Continue cooking until the liquid evaporates. Add the shallots and cook, stirring, about 1 minute.
Add the wine and thyme and continue cooking until the wine evaporates. Add 1cup of the fish stock and continue cooking until the broth is reduced by half.
Add the cream and bring to the boil. Let boil over relatively high heat about 8 minutes, or until the sauce is reduced to about 1½cups.
Line a saucepan with a fine sieve, preferably a chinois. Pour the sauce through, pressing the solids in the sauce with a wooden spoon or spatula to extract as much liquid as possible.
Peel and cut away the pulp from the seeds of the mangoes. There should be about 1½cups pulp. Blend thoroughly in a food processor or electric blender.
Add this pulp to the cream sauce. Add the remaining ¼cup fish stock, the basil leaves, salt, and pepper. Bring to the simmer and let cook about 10 minutes. Set aside.
Preheat the oven to 400 degrees.
Combine the powdered bread crumbs and macadamia nuts in a food processor or electric blender and blend for about 1 minute, no longer. If you overblend, the nuts will lose too much oil.
Sprinkle the fillets with salt and pepper. Dip each halibut fillet in the macadamia nut mixture and coat well on both sides. Pat to make the coating adhere.
Use 1 or 2 ovenproof skillets in which to cook the fish. Heat enough of the remaining oil in the skillet or skillets to barely cover the bottom. Add the fish fillets and cook 1 to 1½ minutes on a side, or until nicely browned. Turn carefully in the skillet. Place the skillet in the oven. Bake5 to 7 minutes, depending on the thickness of the fillets.
Remove the basil leaves from the sauce and reheat it. If necessary, add a little fish stock to thin the sauce to the desired consistency.
Place a fillet in the center of each of 4 warm plates. Spoon the sauce around the fish, and serve.