Combine softened butter, garlic and herbs in a small food processor. Be careful not to run the food processor too long as it will cause the butter to heat up and melt.
Roll the mixed butter into a log in a square of clear plastic wrap
Place in refrigerator to allow to harden
Grill the Steelhead
Bring the grill to 350 degrees on direct heat
Apply olive oil to both sides of the steelhead along with a light coating of salt and pepper, to taste
Place it skin side down on the grill. Grill for 15 minutes.
Flip and cook for an additional 8-12 minutes. Make sure to check the thickest parts of the fish for an internal temperature of 145 degrees. It should be flaky with a fork.
Using two larg spatulas, lift it from the grill and place it on a sheet pan
Add 4-5 pats of chilled butter and allow to melt over the hot steelhead
Dealing with Multiple Thickness of Fish: The drawback of grilling a large fillet like this is that there will be thinner parts and thicker parts. Those will cook at different rates. Move the steelhead's thicker portions to the hottest part of the grill. If you don't want to deal with that, simply cut the fillet into several pieces of similar thickness. Skin Removal: Don't remove the skin before grilling. Not only will the skin helped protect the flesh, but it peels right off after cooking. Herb Mixing: Mix and match herbs. While I used rosemary and thyme, use whatever you have in your garden. The bright, fresh flavors complement the fish perfectly.Grill Flareups: Watch the grill for flareups. Usually, you only need to worry about those with fatty meats, and you won't get anything dripping from the fish, but it is best to closely monitor the grill, just in case.Softened Butter: Make sure to bring the butter to room temperature to mix in the herbs and then put back in the refrigerator to harden. You can make this a few days ahead of time as it can keep in the refrigerator for a couple of weeks. Storage: Leftover fish can be kept in refrigerator in sealed container for up to 2 days.