In a large non-stick pan, over medium heat, melt ½ TBSP of your butter and sauté your diced onions and thyme until soft. Stirring occasionally to not add color to the onions. While they are cooking, roughly chop your walnuts.
In a blender or food processor, add in your cooked onion and thyme, walnuts, dijon mustard and salt. Blend until creamy.
Keeping the skin on, spread the creamy mixture over the top and sides of the halibut, do not put it on the skin side. In a bowl, add your panko breadcrumbs and cayenne pepper.
Gently press your halibut top-side down into the breadcrumb mixture and press the breadcrumbs into the sides of the halibut so that it's completely covered in your panko breadcrumbs.
In a large non-stick pan, over medium heat, melt your second ½ TBSP of butter. Once the pan is hot, place your halibut top-side down and sear until it's a nice golden brown. About 3-5 minutes.
Flip your halibut so that it's skin side down, and place pan into your oven.
Bake at 350F for 7-10 minutes, until halibut is cooked through.