Preheat oven to 350°F (180°C).

Add walnuts to a baking sheet and toast for 7-10 minutes. Allow to cool slightly before chopping.

Turn up oven temperature to 400°F (200°C).

Peel apples and dice into bite-sized cubes.

Add chopped apples to a large bowl with topping ingredients, tossing to combine. Refrigerate or store covered at room temperature until ready to serve.

Slice baguette into ½-inch thick slices and arrange on a lined baking sheet in a single layer. Brush both sides with olive oil and sprinkle with salt. 

Bake in the middle rack of the oven for 6-8 minutes (depending on size), until crispy and golden.

Serve topping with toasted crostini.