Delia's shepherd's pie with cheesy leeks

I've made this a few times and we love it every time. It's a lovely, stick-to-your ribs supper and can be made in stages through the day or altogether, which is useful in a recipe. This time I didn't have the called for turnip or rutabaga (Canadian for swede) but used mushrooms and a red bell pepper instead.

Delia's shepherd's pie with cheesy leeks (adapted slightly from Delia on line)
1 lb ground lamb
1 tbsp olive oil
2 medium onions, chopped
2 medium carrots, finely chopped
a handful of mushrooms, chopped
1 red pepper, finely chopped
1/2 tsp cinnamon
1/2 tsp dried thyme
275 mL vegetable stock
1 tbsp tomato paste
salt and pepper
2 oz sharp cheddar, grated
2 medium leeks, cleaned and cut into half moon slices
2 lbs potatoes
1 tbsp butter
a couple of spoonfuls of greek yogurt, plain
salt and pepper

Gently heat the olive oil in a large saute pan. Add the onions and fry for about 5 minutes until slightly brown. Add the rest of the vegetables and cook for another 5 minutes. Move the vegetables to the side of the pan and add the lamb, cooking it until it is all browned, separating it with a spoon. Season with salt and pepper and mix in with the vegetables. Add the stock and the tomato paste and mix into the mixture. Turn the heat down, cover the pan and let simmer gently for about 30 minutes.

Meanwhile, cook the potatoes until fork tender. Drain the potatoes and let them steam for about 5 minutes. Mash the potatoes with the butter and yogurt, and season with salt and pepper.

Spoon the meat into a baking dish and smooth it out into an even layer. Spread the potatoes over the meat evenly. Sprinkle the leeks over the top as evenly as you can and then top with the grated cheese.

Bake at 400 deg F for about 25 minutes until the cheese is melted and the potatoes are slightly browned.