Press dressing mix evenly onto all surfaces of beef pot roast. Place onions and garlic in 4-1/2 to 5-1/2-quart slow cooker; top with pot roast. Add bell peppers and broth. Cover and cook on HIGH 5 hours, or on LOW 8 hours. Add zucchini. Continue cooking, covered, 30 minutes or until pot roast is fork-tender.