Cut squash in half lengthwise. Discard seeds and membranes. Cut each half into three wedges. In a 5- to 6-quart slow cooker, place squash and onion. Trim fat from chops. Place chops on top of squash and onion in cooker.

In a small bowl, stir together broth, marmalade, mustard, marjoram, and pepper. Pour broth mixture over chops and vegetables in cooker.

Cover and cook on low-heat setting for 5 to 6 hours or on high-heat setting for 2 1/2 to 3 hours. Transfer chops and vegetables to a serving platter, reserving cooking juices; cover chops and vegetables and keep warm.

For sauce, strain cooking juices into a glass measuring cup; skim off fat. Measure juices; add water, if necessary, to equal 1 3/4 cups. Pour juices into a medium saucepan. Stir together cornstarch and cold water. Add to saucepan, stirring to combine. Cook and stir over medium heat until thickened and bubbly; cook and stir for 2 minutes more. Serve sauce with chops and vegetables.