Grease a 3-quart casserole dish, 2 1/2 to 3 inches deep, with 2 teaspoons butter.
Sprinkle 1/4 cup Parmesan into dish to lightly coat bottom and sides, and set dish aside. Separate egg whites from yolks, setting whites aside in a mixer bowl and reserving only 4 of the 6 egg yolks.
Melt remaining butter in a large nonstick skillet over medium heat and add mushrooms and next 5 ingredients. Cook 4 minutes, stirring constantly.
Sprinkle flour over vegetable mixture, and cook, stirring, 1 minute. Slowly add milk, and bring to a boil, stirring constantly, until mixture thickens.
Remove from heat; whisk 4 reserved egg yolks into sauce, one at a time, until blended. Stir in crabmeat and chives.
Beat egg whites in mixer bowl on high speed until stiff and billowy but not dry. Stir one-fourth of the whites into crab mixture to loosen; fold in remaining whites until just combined (be careful not to deflate the volume).
Transfer mixture to prepared dish; sprinkle top with remaining 1/4 cup Parmesan cheese.
Bake at 400 for 30 minutes, or until puffed and golden and a knife inserted into the center comes out clean.
Sprinkle with parsley.
Serve immediately.
Casserole Basics: Glass and ceramic (earthenware) dishes work best for casserole cooking because they heat up quickly and evenly. If you substitute a metal baking pan, the casserole usually cooks more slowly, so you may have to increase the oven temperature by 25 to compensate.
Always start with the freshest seafood available and pat it dry. If you use individual steaks or fillets, select pieces that are at least 1 inch thick. If a single fillet from a large fish tapers to a thin end, fold the thin end under to help the fillet cook uniformly.
If the fillet in the casserole is thick, let it sit at room temperature while you prepare the other ingredients for the dish. It will cook more quickly.
Thick fillets or steaks, especially if covered with a sauce or topping, can take from 12 to 15 minutes of baking time per inch. To check for doneness, cut into the fillet at the thickest part to see if it's opaque inside and pulls apart with little resistance.
Casserole recipes calling for fully or partially cooked seafood or fish need only enough time in the oven to get hot, so an instant-read thermometer comes in handy. If the middle of the casserole registers 160, it's done.
If you're making a casserole with cooked seafood that's added to a sauce, heat the sauce separately, stir in cold, cooked fish, and then assemble the dish. This will speed up the time in the oven and keep the fish from disintegrating.
Don't cook fish until it flakes easily-- by that point, the fish has given up all of its juices, rendering it dry and leaving a lot of water in the dish. If this occurs, spoon the juices back over the fish when serving.
Remember, the casserole will continue cooking once it's pulled from the oven, so it's better to take it out sooner rather than later.
