Rocket & Watercress Soup

1 medium brown onion, diced
4 cloves garlic, peeled & smashed
Olive oil
2 large bunches watercress, washed and dried
2 large bunches wild rocket, washed and dried
2 stalks of parsley with leaves
ΒΌ cup roughly chopped chives
1 litre chicken stock
2 cups water
250 mls thickened cream (plus extra to serve)
Parsley sprigs, to garnish
Sea salt & freshly ground black pepper to taste