Trim excess fat from meat. Cut meat into 1-1/2-inch pieces.
In 3-1/2 or 4 quart slow cooker and place meat, onions, and mushrooms. Sprinkle with garam masala, ginger, salt, pepper and garlic. Add beef broth.
Cover and cook on low heat setting for 8 to 10 hours or on high heat setting for 4 to 5 hours.
If using low-heat setting, turn to high-heat setting. In small bowl, combine yogurt and cornstarch. Stir yogurt mixture and summer squash into cooker. Cover and cook for 15 minutes more or until sauce is slightly thickened.
Mix cardamom, coriander, cumin, black pepper, cloves, cinnamon, and nutmeg.