Chicken soup and matzo balls

INGREDIENTS SERVES 6 – MAKES ABOUT 2L

For the soup: 2kg chicken thighs and legs; 5 large onions, skins left on, halved, cutting off the rooty bit; 8 carrots, sliced about 2-3cm thick; 4 celery sticks, with leaves, halved; 1 leek, halved; ½ swede; 2 tbsp Telma Chicken Soup Mix (available from kosher shops or online), or 2 good quality chicken stock cubes; 1tsp whole black peppercorns; 1tsp salt