Place pecans on an ungreased baking sheet. Toast at 350 degrees F (175 degrees C) for 5 to 8 minutes.
Peel the squash, and remove the seeds. Cut into 1/2 inch cubes. There will be about 6 cups squash.
Melt butter or margarine in a heavy large skillet over low heat. Add onion and saute until very tender, about 15 minutes. Add squash and toss to coat. Cover. Cook until squash is tender but still holds it shape, stirring frequently, about 15 minutes. Season to taste with salt and pepper. Can be prepared 4 hours ahead. Let stand at room temperature. Rewarm over medium heat before continuing.
Stir in half of the pecans and half of the parsley. Transfer mixture to bowl. Sprinkle with remaining pecans and parsley. Serve.