Shepherd's Pie: a definitive version of the classic family recipe

Ingredients:
1 Tbsp Cooking Oil.
25g Butter.
1 Diced Onion.
1kg Ground Lamb.
30g Flour.
2 Tsp Dried Rosemary.
1 Tbsp Tomato Ketchup.
1 Tsp Paprika.
Salt and Pepper.
175g Diced Carrot.
175g Peas.
1.2kg Potatoes: Peeled and diced.
175g Sharp Cheddar Cheese.
50ml Cream.
1 Egg Yolk.

Instructions:
Heat the oil and butter in a large frying pan over a medium high heat.
Fry the onion until softened.
Bring some water to the boil and salt generously and use to boil the potatoes.
Turn up the heat to high and fry until brown which should take around 10 minutes, if you need to drain off any liquid it gives off (especially important if your lamb has been frozen), you want to fry and not boil the lamb.
Turn down to medium and add the flour and cook for 3-4 minutes.
Stir in the rosemary, ketchup and paprika before adding in 500ml of water and cooking out until the mixture becomes very thick.
Add in the peas and carrots and cook for 3-4 minutes.
Layer this meat mixture in to a large casserole bowl (32cmX25cm or so).
By now your potatoes should be cooked and ready to be mashed.
Take mash and add in the cheese, cream and egg yolk and stir together.
Layer your mash over the meat mixture and use a fork to flatten and rough up the surface.
Bake in the oven at 180°C or 350F for 40 minutes.