Brisket With Sweet Potatoes

• 2 stalks celery (finely)
• 2 carrots (finely chopped)
• 1 onion (finely chopped)
• 1 chunks
• 1 pitted prunes coarsely
• 1,600 gr beef brisket (boneless)
• 575 gr sweet potatoes
• 475 ml beef stock
• 240 ml water
• 130 gr parsnips