Place two racks in the middle and upper third of the oven. Set the oven to preheat to 375°F so that it gets ready for baking.
Use a food processor to grind four slices of the white sandwich bread finely. Let the food processor grind it till it turns into finely ground material with the help of the blade attachment. Once done, transfer it to the large bowl.
Add coarsely chopped onion, garlic cloves, coarsely chopped carrot, and celery stalk to the food processor until finely chopped. Add leaves from 1 small bunch of fresh parsley till 1/2 packed cup and finely chopped two tablespoons of leaves followed by refrigeration for garnishing. Transfer the chopped veggies to the bowl of bread crumbs.
Use the box grater to grate the mozzarella cheese on the large holes, about 1 cup, and keep it reserved for sparkling. Similarly, use the small holes of the box grater to grate Parmesan cheese till 1/2 packed cup.
Add ground beef, Parmesan, egg, marinara sauce, kosher salt, Worcestershire sauce, black pepper, Italian seasoning, and red pepper flakes. Mix the ingredients with your hands until they combine. Transfer the mixture to the loaf pan and shape it with your hands.
Put the loaf pan in the preheated oven. Let it remain there until cooked through for almost 1 hour and 10 minutes. You can place a thermometer inserted in the center to get an idea of the instant temperature, and it should register at least 165ºF.
Once the meatloaf achieves at least 160ºF, remove it from the oven. Now carefully drain or spoon off the excess fat in the baking dish. Top the meatloaf with 1 cup of marinara sauce while putting the remaining around the sides. Use the reserved mozzarella cheese and sprinkle it over the meatloaf. Put the oven to broil on high.
Let the cheese turn golden brown and melt by placing it on the upper rack. It will take about 3 to 4 minutes. Now sprinkle the remaining parsley over the top of the meatloaf. Slice the loaf into pieces and serve it with marinara sauce on the top.