Place raisins in a small bowl. Cover with boiling water; let stand for 5 minutes.
Meanwhile, in a large bowl, combine the oats, brown sugar, pie spice and salt. Stir in the milk, apple and butter. Let stand for 5 minutes. Drain raisins; stir into oat mixture.
Transfer to an 8-in. square baking dish coated with cooking spray. Sprinkle with walnuts. Bake, uncovered, at 350° for 30-35 minutes or until a knife inserted in the center comes out clean.
Read all ingredient labels for possible gluten content prior to use. Ingredient formulas can change, and production facilities vary among brands. If you're concerned that your brand may contain gluten, contact the company.