Prepare the marinara sauce: in a saucepan, heat olive oil to medium heat. Sauté the onion and garlic until fragrant, about 1 to 2 minutes. Add the rest of the marinara ingredients. Bring to a boil, and then reduce to a simmer. Simmer about 10-15 minutes.
Meanwhile, prepare the chicken. Slice each chicken breast in half, horizontally, so that you have 2 cutlets (1/2 inch thick) per breast. Pound each cutlet slightly until evenly thick. Pat dry, and season both sides with some salt and pepper. Don't overdo it, the breading will be seasoned too.
In shallow bowl, add the flour and season with a generous pinch of salt and black pepper. Then, in a second shallow bowl, add the whisked eggs. In a third shallow bowl, add the Italian breadcrumbs, grated parmesan, 1/2 teaspoon Kosher salt, and the garlic powder.
Heat some oil in a large non-stick skillet.
One by one, dredge chicken on flour, then egg, then bread crumbs (in this order), shaking any excess ingredients as you go. Repeat with remaining chicken breasts.
Test that the oil is hot enough by throwing in some breadcrumbs. If you hear a sizzle, then it's ready. If you add the chicken to the pan before the oil is hot enough, you'll end up with a soggy crust.
On an oven-proof non-stick skillet, heat some oil to medium heat. Cook chicken, about 5 minutes each side, until golden brown and cooked through. Quick tip: before one side is golden brown, I flip it and cook the other side, and flip it back again and so on. This promotes even cooking, while preventing one side from browning too quickly.
Turn off heat when chicken is ready. Add a handful of shredded mozzarella cheese on top of each chicken cutlet, without piling it high (for even melting). Broil in the oven for 1 to 3 minutes, until melted. Important: If your skillet is not oven-proof, transfer to a sheet pan and broil. Watch it carefully, don’t let it burn.
Top with a few spoonfuls of marinara sauce and parsley. Serve with spaghetti or any other sides you like!