Preheat oven to 425 degrees. Place bread cubes on a baking sheet and toast in the preheated oven until they become crisp, about 7 minutes. Remove from the oven and transfer toasted croutons to a bowl. Grease or line the baking sheet with foil.

Cut the squash in half lengthwise. Use a spoon to scoop out the seeds and their stringy connective tissue. Add salt and pepper to the inside of each squash boat. Place each boat cut side up on a greased or foil-lined baking sheet.

Melt butter in a saucepan over medium heat. Add onions, stir and sauté until they turn translucent, about 7 minutes. Stir in spinach and cook until it begins to wilt, about 3 minutes. Remove pan from heat and transfer vegetables to the bowl with croutons. Add crumbled blue cheese and diced apples. Stir to combine.

In a small saucepan, toast walnuts over low heat for about 4 minutes, stirring occasionally. Remove from heat, stir in maple syrup. Let cool for 1 minute. Add candied walnuts to vegetables, cheese and apples. Mix to combine and season with salt and pepper.

Reduce oven temperature to 400 degrees. Place squash in the oven for 8 minutes, then remove and fill each squash half with stuffing mixture, dividing evenly. Return stuffed squash to the oven for 40 minutes. Remove and garnish each boat with parsley.