Preheat the oven to 350˚F (180˚C). Grease 2 8-inch (20 cm) round cake pans with nonstick spray.
Heat the oil in a medium pan over medium heat. Add the celery, onion, and carrot and sauté until translucent, about 10 minutes. Set aside to cool.
In a large bowl, combine the ground beef, Dijon mustard, paprika, Worcestershire sauce, tomato paste, eggs, panko, milk, and salt (if using). Stir to combine, add the cooled sautéed veggies, and mix until homogeneous.
Divide the meatloaf between the prepared pans and spread the mixture evenly.
Bake for 20-25 minutes, until golden brown. Let cool for 30 minutes.
Add 1 meatloaf layer to a serving plate. Spread a thin layer of tomato paste over the top. Add 2 cups (500 g) of mashed potatoes to the center and spread evenly. Top with the other meatloaf layer and spread with a layer of tomato paste and 2 cups (500 g) mashed potatoes. Use an offset spatula to cover the sides of the “cake” and spread the potatoes over the whole cake in an even layer.
Add another cup (250 g) of mashed potatoes to the top of the cake and spread evenly over the whole cake.
Add the remaining mashed potatoes to a piping bag fitted with a star tip. Pipe rosettes around the base and top of the cake.