Swedish-style mini-meatballs

INGREDIENTS

• 2 tbsp olive oil
• 1 Spanish onion
• ½ tsp dried oregano
• ¼ tsp celery
• pinch cumin
• 500g/1lb 2oz pork mince
• 1 free-range egg yolk
• 1 tbsp plain flour
• 1 tsp mustard
• salt and freshly ground black pepper
• 1 tbsp olive oil
• 1 clove fresh garlic
• 250ml/9fl oz passata
• hard cheese (such as gruyère, emmental