Peppery Mozzarella Salad

• 2 beefsteak tomatoes
• 1 ripe avacado
• 80 gr mozzarella cheese
• 80 ml olive oil
• 30 ml lemon juice
• 6 gr green peppercorns
• 2 ml dry mustard
• 1 gr oregano (dried)
• 2 slices shallots