Preheat the oven to 450 degrees & prepare a sheet pan with foil, parchment, or a silicone mat.
Melt butter in a skillet or saucepan over medium heat. Add the shallot and cook on medium heat until the shallot is soft and translucent, but not browned. 2-3 minutes
Add the garlic, cook for another minute.
Remove from heat and add the lemon zest and lemon juice, and stir to combine.
Place the steelhead on the sheet pan. Season the top with salt & pepper. (If the skin has been removed, you may season lightly on both sides)
Pour the shallot mixture over the fish, taking care to keep as many of the shallots on top of the fish as possible.
Bake at 450 for 13-16 minutes, depending on the size of the fish. The fillet pictured cooked perfectly in 14 minutes in my oven. To test, try flaking a bit of the thickest part of the fish. We love it medium – just barely set in the middle – if you prefer it cooked all the way through, aim for the higher time. Allow to rest 5-10 minutes before serving.
If serving formally, transfer to a platter and garnish with fresh parsley and a squeeze of lemon. For casual meals, just garnish and serve right from the pan.