Meatball Sandwich with Pesto

Ingredients

• 50 g (1.8 oz) bread
• 80 ml (1/3 cup) milk
• 500 g (1.1 pound) ground beef
• 1 onion
• 2 tbsp parsley
• 3 garlic cloves
• 1/2 tsp oregano
• 1 tsp salt
• 1 egg
• 1 tbsp ricotta (or mascarpone, or cream cheese)
• 1 tsp olive oil, to drizzle
• 2 tbsp olive oil
• 1/2 celery stalk
• 1 carrot
• 1 anchovy (optional)
• 1 tsp sugar (optional)
• 1 tsp all-purpose flour
• 500 g (2 1/4 cups) tomato passata
• 250 ml (1 cup) water
• 3 basil leaves
• 1 sprig of parsley
• 2 sourdough breads (about 400g/1 pound each)
• 2 tbsp Pesto Genovese
• 250 g (1/2 pound) sliced Mozzarella cheese