Re: I need a Steelhead/Coho recipee that isn't just seasoned fish
Scipper: Personally, I don't see too much difference between feisty steelhead and atlantic/landlocked salmon as far as prep for good eats. The steelhead flesh is slightly more firm with more red and less orange to the coloration.
Both salmon and steelhead are tremendous smoked- Apple, alder, or cherry are great with these guys when the fish is properly and carefully brined, and then very carefully low-temp smoked under a watchful eye. However, you were looking for a recipe that isn't seasoned fish.
First a comment on buttermilk: this and some other "soak-it" methods are most common as a way of softening bones like when a person doesn't know how to sharpen a fillet knife to a razor edge and they want to zap the bones in a pike or big pickerel. Marinades can add flavor and some people like that, but the primary reason to marinade is to expose a tough meat to something acidic to tenderize it (vinegar in dressings, similar effects of wine marinades) and perhaps add a touch of acidity to the palette as well.
Steelhead and landlocked atlantic salmon, being oily but not fishy-tasting fish, can be prepared as a distinctive entree very simply in one of these two ways with many variants possible:
Smoker: coat fillets with olive oil and a bit of real maple syrup brushed on and low-temp smoke with alder or apple for about 45 mins. Remove from smoker, brush with melted butter that has had a few cloves of crushed fresh garlic steeped in it (don't get hot enough while simmering butter that garlic gets toasted or browned) and cooked on greased grill until flaky. Dust with fresh-ground white peppercorns (lightly!!) and serve. You can snow it with a little basil if you like.
Melt butter and simmer a few minutes with fresh garlic- don't toast the garlic! The goal is to infuse the garlic essence in to the butter. Brush on both sides of skinless fillets. Take a pinch or two of Lawry's Perfect Blend and drop on to fillets. Lightly dress with dried basil. Place in glass flat-bottomed dish with plenty of the butter on the bottoms. Add a very little white wine to bottom of glass dish. Place in pre-heated 500F oven for 5-7 minutes turning oven down to 325F when you put fillets in oven. Fish is done when flaky and a little of the natural oils are beginning to "sweat" from the fish. Alternatively, you can do the same thing in a cast-iron pan - serve up the fish, splash a little cream and white wine in the cast iron with some corn starch and in about 30 seconds you will have a wee bit of a sauce to dress some egg noodles with and serve while fish is still hot!