Preheat oven 350°F, and line a baking sheet with parchment paper.
Heat olive oil in a large skillet over medium-high heat.
Add in onion and sauté, stirring occasionally until golden, about 8 minutes.
Add in garlic and cook, stirring often, 2 minutes more.
Remove from heat and let mixture cool slightly.
Meanwhile, bring a large pot of salted water to a boil over medium heat.
Add in potatoes and cook, uncovered, about 15 minutes or until completely tender.
Transfer potatoes back to pot and mash until smooth.
Add in 3/4 cup milk and butter, and fold in with a spatula until well combined.
Liberally season with salt to taste.
In a large bowl, mix together breadcrumbs and remaining 1/4 cup milk, and let sit 5 minutes to soften.
Add in onion and garlic mixture, along with herbs, lemon zest, tomato paste, Worcestershire sauce, mustard, egg, carrots and ground beef.
Season with a large pinch of salt, and mix together with hands or a spoon until just combined.
Place 2 slices of prosciutto in each cup of 2 standard size muffin tins, making sure to fully cover the bottom and the sides, leaving about 1/2 inch overhang on all sides.
Firmly press about 1/4 to 1/3 cup ground beef mixture into each prosciutto-lined mold.
Bake in oven 20 minutes or until just cooked through.
Remove meatloaves from tins and place on the prepared baking sheet.
Top each meatloaf with 3 tablespoons mashed potatoes.
Grate Parmesan cheese over each meatloaf and sprinkle with pepper.
Preheat broiler to high and broil for 8 to 10 minutes until the tops are golden.
Serve immediately with roasted tomato jam or ketchup, if desired.