Preheat the oven to 425 degrees.
Place the sliced potatoes on a large sheet pan. Drizzle with 1 tablespoon olive oil, ½ teaspoon kosher salt and ⅛ teaspoon black pepper. Toss to coat and arrange in one even layer on the sheet pan. Bake for 12 minutes.
While the potatoes bake, make the sauce. Blend together all ingredients until smooth in a small blender, food processor or using an immersion blender. For a thicker, creamier sauce, use more Greek yogurt. For a runnier, dressing-like consistency, use less Greek yogurt or add 1-2 tablespoons of water. Refrigerate for up to 3 days in a sealed container or jar.
Remove the sheet pan from the oven and flip the potatoes, moving them to the other half of the baking sheet (it’s ok if they overlap a bit).
Add the asparagus and salmon to the open half of the sheet pan. Drizzle the asparagus with ½ tablespoon olive oil, ¼ teaspoon kosher salt and ⅛ teaspoon pepper. Toss to coat.
Season the salmon with ½ teaspoon kosher salt and ¼ teaspoon pepper. Place the butter on top of the salmon and top each piece of butter with a slice of lemon.
Bake for another 12 minutes or until the salmon is cooked through and can be easily flaked with a fork.
Crumble the feta cheese on top of the asparagus and potatoes. Drizzle everything with the Green Goddess sauce and season to taste.
