Prepare Lemon Zest and Juice: Zest the lemon using a zester and set the zest aside. Cut the lemon in half and squeeze the juice into a small bowl.
Make the Walnut Crunch Topping: In a medium bowl, combine the chopped walnuts and panko bread crumbs. Melt 1½ tablespoons of the garlic herb butter and mix it into the crumb mixture until evenly coated and slightly clumpy.
Arrange the Salmon: Line a baking sheet with aluminum foil. Place the salmon fillets skin-side down on the prepared baking sheet, leaving a little space between each piece.
Prepare the Lemon Mayo: In a small bowl, whisk together the mayonnaise, lemon juice, and lemon zest until smooth.
Coat the Salmon: Spread a thin, even layer of the lemon mayonnaise mixture over the top of each salmon fillet using a spoon or spatula.
Add the Walnut Crust: Press the walnut and panko mixture onto the mayonnaise-coated tops of the salmon fillets so it forms a thick, even crust.
Bake the Salmon: Place the baking sheet in a preheated 400°F oven and bake for 10 to 12 minutes, or until the salmon is cooked through and the crust is golden and crisp.
Prepare the Potato Shells: Slice the baked russet potatoes in half lengthwise. Carefully scoop most of the potato flesh into a large bowl, leaving a thin layer so the skins keep their shape.
Make the Colcannon Filling: Add the shredded cabbage and chopped green onions to the bowl with the potato flesh. Add the remaining ½ tablespoon garlic herb butter, salt, and black pepper. Mash and mix until the potatoes are creamy and the vegetables are evenly distributed.
Refill the Potatoes: Spoon the potato mixture back into the potato skins, mounding it slightly so the filling sits above the edges.
Add Cheese: Sprinkle shredded white cheddar cheese evenly over the tops of the filled potatoes.
Bake the Potatoes Again: Place the stuffed potatoes on the baking sheet and bake at 400°F for about 15 minutes, until the cheese is melted and lightly golden.
Serve: Remove the salmon and potatoes from the oven. Let them cool slightly, then serve the crunchy walnut salmon alongside the twice-baked colcannon potatoes.