Preheat the oven to 425˚F. Line a baking sheet with foil and spray with cooking spray. Set aside.
Place the salmon on the foil, skin side down, and top with walnuts. Press them gently on the salmon.
Bake the salmon in the preheated oven for 6-12 minutes.
Meanwhile, in a small saucepan whisk together the Guinness, honey, apple cider vinegar, dried thyme and garlic powder on the stove over medium-high heat. Stir frequently until it coats the spoon. Remove from heat. It should thicken as it cools. The consistency should end up being that of maple syrup.
Once the salmon is cooked, drizzle the reduction over the salmon.