Preheat oven to 350 degrees. Combine oats, walnuts, flour, salt, and sugar in a large bowl. Melt butter in a small saucepan over medium heat. Whisk in honey. Stir into oat mixture.Advertisement
Spread streusel on a rimmed baking sheet lined with a nonstick baking mat. Bake 12 to 15 minutes. Let cool on baking sheet on a wire rack.
Do not wait for the streusel to get crunchy in the oven; that will happen when it cools and dries after it is removed from the oven.