grog's shepherd's pie

This recipe is our typical way of using up the rest of a roast leg of lamb. The quantities are sufficient for about 4 people. The original probably came from Georgina Horley's “Good food on a budget”, but we have been modifying it for some time.

The percentages below relate to the real weight of the meat.

Ingredients

quantity	ingredient	step
100 g	onion, minced or finely chopped	1
50 ml	oil or bacon fat, for frying	1
8 g	garlic, crushed	1
300 g	cooked lamb meat	2
50 g	dried toast	2
1 g	thyme (fresh), chopped	3
1 g	rosemary (dried)	3
0.6 g	tarragon (dried)	3
8 g	salt	3
	freshly ground pepper	3
240 ml (80%)	gravy/liquid (see discussion)	3
480 g (160%)	mashed potatoes (see discussion)	4
	butter flakes (optional)	4

Preparation

Gently fry onion in oil or bacon fat, add garlic, fry another 30 seconds.
Mince meat, fried onion and bread.
Put mixture in a baking dish, add thyme, tarragon, rosemary, salt and pepper, and gravy, mix well.
Make mashed potatoes according to directions, but don't add salt. Spread over meat mix. Optionally, sprinkle with some butter flakes or brush over egg yolk with water.
Bake in 180° (or warmer?) oven for 20 to 30 minutes, maybe with top heat to brown the surface.

Discussion

Apart from the quantities, I need to consider the area of the baking dish. I've decided that there should be 1.2 g and 1.5 g of meat per cm² of dish, so for the 300 g in the recipe I would need a dish between 200 cm² and 250 cm² in size.

The quantity of gravy appears critical. At one point I thought that the weight should match the weight of the meat, but that's too much.

I've had wildly different quantities of mashed potato in the past, between 50% and 200% of the weight of the meat.

Maybe it would make more sense to bake at a higher temperature, possibly with grill, so that the gravy doesn't boil before the surface browns.