Pastel de papa (Argentinian cottage pie)

Ingredients

• 3 tbsp olive oil
• 3 medium onions, chopped
• 1 red pepper, chopped
• 1 garlic clove, chopped
• 1 tsp chilli flakes
• 1 tsp sweet paprika
• 1kg (2.lb) beef mince
• 400ml (1 2/3 cups) tomato passata
• 1 tsp salt
• 1 tsp ground black pepper
• 4 hard-boiled eggs, peeled and chopped
• 2kg (4½lb) potatoes, peeled and cut into medium-sized pieces
• 110g (½ cup) unsalted butter, diced
• 120ml (½ cup) milk
• a pinch of ground nutmeg
• freshly grated Parmesan cheese