Garbanzo Shepherd's Pie

• 1 onion
• 1 mashed potatoes
• 525 gr garbanzo beans (cooked)
• 475 ml low-fat soy milk
• 300 gr peas (frozen)
• 190 gr carrots (diced)
• 120 ml water
• 80 gr mushrooms
• 75 gr celery (diced)
• 5 gr cornstarch mixed w