Preheat oven to 350 degrees.

Peel potatoes and cut into quarters. Add to a large pot, cover with water, add salt, and heat to boiling. Reduce heat and cook until potatoes are soft. While the potatoes are cooking, make the meat filling.

Add a splash of olive oil to a large heavy pot and heat over medium heat until shimmering. Add ground lamb, stir to break into small pieces, and cook until meat is brown. If a large amount of fat is rendered from your meat, spoon some out, leaving behind around 1/4 cup. Remove the browned meat from the pot, and add onion, carrots, and celery to the oil that remains behind. Cook these, stirring often, until the onions begin to sweat and the vegetables all begin to soften. When a fork will easily pierce a carrot piece, it's time to move to the next step. 

Add back in the meat and stir all to combine. Make a clear space in the center of the pot, and add the garlic, rosemary, and thyme to that spot. Toast about 30 seconds, then stir into the rest of the ingredients. Cook for about a minute. Make another clear space in the center of the pot, and add the tomato paste, red wine, and Worcestershire sauce. Stir to combine, and then mix throughout the meat mixture. Sprinkle the flour over the mixture and cook, stirring, until the whiteness of the flour has disappeared. (Stir well, so the flour doesn't stick to the bottom of the pot and burn.)

Add the beef stock and frozen vegetables. Stir well, and heat on medium heat until bubbling. Return to low heat, and allow to simmer while you prepare the mashed potatoes. Stir occasionally, and add a bit more beef stock if your filling is dry.

Drain the potatoes, and transfer to a bowl for mashing. Add butter and mix a bit to melt the butter. Then add milk and vigorously mash the potatoes. (You can use an electric mixer if you like them very very smooth.) Then add the egg yolk and cheese, stirring these into the mashed potatoes with a spoon. Mix well until smooth and uniform.

Spray a little food release spray in the bottom of a 9"x13" casserole pan. Pour in the meat mixture, and spread evenly. Dollop the mashed potatoes on top of the meat mixture, and then use a spoon to spread them out. Reaching the edges and covering all the meat is desirable, but not necessary. 

Optional: throw some cheese on top? Shredded parmesan is a nice choice. Traditional Shepherd's Pie wouldn't have cheese... but it's possible to improve upon tradition, right?

Bake your Shepherd's Pie for 15 minutes, until the mixture is bubbling about the edges of the potatoes. If you like a crispy top, increase the oven temp to 400 degrees, move the pan up a level in the oven, and bake another 5-10 minutes until it looks nice.

Remove from the oven, and let the pie rest for about 10 minutes before cutting (if you want pretty slices that hold together somewhat on the plate; if you don't care about that, serve right away!)