Cheese & Potato Pie | Old School Cheese, Onion & Potato Pie

Ingredients

• 1 vegetable stock cube
• 1 kg potatoes, peeled and chopped into 2.5cm pieces
• 1 tbsp. olive oil 
• 35 g butter 
• 1/2 leek, finely diced 
• 1 onion, finely diced 
• 1 tsp. salt 
• 1/2 tsp. black pepper 
• 30 g plain flour 
• 250 mL milk 
• 180 g extra mature cheese
• 1 tbsp. Dijon mustard
• 1/4 tsp. ground nutmeg 
• 1/2 tomato, sliced (optional)