01 - Set oven temperature to 400°F and line a baking sheet with parchment paper.
02 - Heat olive oil in a saucepan over medium heat. Sauté minced garlic and chopped onion until softened, about 3 minutes. Add crushed tomatoes, dried basil, oregano, salt, black pepper, and sugar. Simmer uncovered for 15 to 20 minutes, stirring occasionally. Stir in fresh parsley if using and keep warm.
03 - Using a meat mallet, gently flatten chicken breasts to an even thickness between 0.6 and 0.8 inches. Season both sides with salt and black pepper.
04 - Set up three shallow dishes: place flour in the first, whisk together eggs and milk in the second, and combine Italian-style breadcrumbs, 1/2 cup grated Parmesan, garlic powder, and dried oregano in the third.
05 - Coat each chicken breast first in flour, then dip into the egg mixture, and thoroughly cover with breadcrumb mixture.
06 - Heat 1/4 cup olive oil in a large skillet over medium-high heat. Fry chicken breasts for 3 to 4 minutes per side until golden brown but not fully cooked. Transfer to paper towels to drain excess oil.
07 - Place fried chicken breasts onto the prepared baking sheet. Spoon marinara sauce generously over each piece. Sprinkle shredded mozzarella and additional grated Parmesan cheese on top.
08 - Bake in preheated oven for 15 to 18 minutes, or until chicken reaches an internal temperature of 165°F and cheese is melted and bubbly.
09 - Meanwhile, cook spaghetti in salted boiling water according to package instructions until al dente. Drain well.
10 - Plate spaghetti, top with additional marinara sauce, then place one chicken breast over the spaghetti. Garnish with fresh basil or parsley if desired.