Savoy cabbage soup with meatballs (quick and easy)

Ingredients

• 1 small savoy cabbage
• 1 large leek
• 1 red onion
• 1.75 litres of water
• 2 tablespoons white miso paste
• vegetable stock cube (optional)
• juice of half a lemon
• 400 grams of tomatoes
• 150 grams of ground meat (pork and beef)
• 1 teaspoon mustard (Dijon)
• salt and pepper to taste
• small handful of breadcrumbs
• olive oil