Drop wild and brown rice into boiling water in large saucepan.  Add salt, cover and return to boil.  Simmer gently for 50 minutes or until tender and most of water is absorbed.  While rice is cooking, melt butter in skillet, add onion and sauté gently until soft, about seven minutes.  Combine onion, pistachios, parsley, sherry, tarragon and cooked rice; stir to mix.  Turn into well-buttered 9- or 10-cup casserole or ring mold.  Bake, covered, at 350°F. until heated through (30 minutes for casserole or 25 minutes for ring mold).  If using ring mold, gently invert onto platter to serve.  If necessary, loosen any rice that sticks to mold and use to repair shape of ring.