Toast the walnuts in a dry 3-quart / 2.8 L saucepan over medium heat for 3 minutes, stirring often, until fragrant and a shade darker. Tip them onto a cutting board and chop roughly.
Melt the butter in the same saucepan over medium heat. Add the diced apples, cinnamon and maple syrup. Cook for 5 minutes, stirring, until the edges soften and the juices turn syrupy but the pieces still hold their shape.
Pour in the milk, vanilla and salt. Bring to a gentle simmer, about 2 minutes. Stay close, because milk climbs the pan fast.
Stir in the couscous, cover with the lid and take the pan fully off the heat. Let it stand for 5 minutes, until the milk is absorbed. Fluff with a fork.
Spoon into 4 bowls. Top with the chopped walnuts, a small drizzle of maple syrup and a pinch of cinnamon. Serve warm.