Peel the potatoes and cut into 1/2-inch dice. Place in a large saucepan and cover with cold water and some salt. Set over high heat, cover and bring to a boil. Once boiling, uncover, decrease the heat to maintain a simmer and cook until tender, approximately 10 to 15 minutes.
Place the light cream (begin with just 1/2 cup, using more if necessary) and butter into a microwave-safe container and heat in the microwave until warmed through, about 35 seconds. Drain the potatoes in a colander and then return to the saucepan. Mash the potatoes and then add the light cream, butter, salt and pepper and continue to mash until smooth. Stir in the yolk until well combined.
For The Pie Filling
While the potatoes are cooking, prepare the filling. Place the olive oil into a 12-inch saute pan and set over medium high heat. Once the oil shimmers, add the pearl onions and carrots and saute approximately 6 to 8 minutes. Do not be afraid to scorch them a bit, but keep them moving in the pan so as not to outright burn.
Add the garlic and stir to combine. Add the mince, salt and pepper and cook until browned and cooked through, approximately 3 minutes.
Grate the fresh nutmeg, about one half a seed, over the meat and stir to incorporate. Sprinkle the meat with the flour and toss to coat, continuing to cook for another minute or two to cook off the flour taste. Make a well in the center/hot spot of the pan. Add the tomato paste to the well and allow it to scorch. You may need to move it around just a bit, but allow a good scorch, all the while keeping it to the center hot spot of your pan.
For The Gravy
When your tomato paste has begun to brown up a bit, aka the burn, incorporate the pearl onions and diced carrots with the paste. Add the Worcestershire and thyme, and stir to combine. Remove the pan from the heat. Add in the Marsala and stir. Place pan back atop heat and add the chicken broth and stir. Bring to a boil, reduce the heat to low, cover and simmer slowly 10 to 12 minutes or until the sauce is thickened slightly. The sauce will continue to thicken during the bake.
Add the peas to the meat mixture and spread evenly into an 11 by 7-inch baking dish. Top with the mashed potatoes, starting around the edges to create a seal to prevent the mixture from bubbling up and smooth with a rubber spatula. If desired, using the tines of a fork, comb parallel lines over the top of the mash. The peaks and valleys will brown unevenly during the bake and come out beautifully unbalanced, rustic, and golden.
Place on a foil lined half sheet pan on the middle rack of the oven and bake for 25 minutes or just until the potatoes begin to brown. Casserole will likely bubble up and over, so I definitely recommend using the pan beneath during the bake. Remove to a cooling rack for at least 15 minutes before serving.