Preheat the oven to 350 degrees F (175 degrees C).

Place pecans on an ungreased baking sheet.

Bake pecans in the preheated oven until toasted, 5 to 8 minutes.

Peel squash and remove seeds. Cut into 1/2-inch cubes to yield about 6 cups squash.

Melt butter in a heavy large skillet over low heat. Add onion and sauté until very tender, about 15 minutes. Add squash and toss to coat. Cover and cook, stirring frequently, until squash is tender but still holds its shape, about 15 minutes. Season with salt and pepper.

Stir in 1/2 of the pecans and 1/2 of the parsley. Transfer mixture to a bowl. Sprinkle with remaining pecans and parsley, then serve.