Oyster And Artichoke Soup

• 16 ooysters divided
• 3 strong homemade stock
• 1 clove garlic (crushed)
• 1 onion (chopped)
• 300 ml water
• 45 gr butter
• 24 gr flour
• 1 gr cayenne pepper
• 1 gr marjoram fine (fresh chopped)
• 1 package concentrated chicken broth