Cut the spaghetti squash in half and remove the seeds. Wrap each half in foil and roast in the oven for about 40 minutes, or until tender in the middle.
Remove from heat, let cool slightly and scrape the squash into a bowl using a fork. Combine 4 cups spaghetti squash with the red grapes, walnuts, olive oil and vinegar and stir to combine. Season with freshly ground pepper. Reserve any additional squash for a later use.
If desired, place squash on radicchio leaves for a colorful presentation. Serves 6