01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Place each chicken breast between two sheets of plastic wrap and flatten to a uniform 1/2-inch thickness using a meat mallet or rolling pin.
03 - Set up three shallow bowls: one with flour, one with eggs whisked with milk, and one with breadcrumbs combined with Parmesan, garlic powder, oregano, salt, and pepper.
04 - Dredge each chicken breast in flour, shaking off excess, then dip into egg mixture, and finally coat thoroughly with breadcrumb mixture.
05 - Heat olive oil in a large skillet over medium-high heat. Fry chicken breasts 2-3 minutes per side until golden brown. Transfer to prepared baking sheet.
06 - While the chicken bakes, heat olive oil in a saucepan over medium heat. Sauté garlic and onion until softened, about 3 minutes. Add crushed tomatoes, basil, oregano, sugar, salt, and pepper. Simmer uncovered for 15 minutes, stirring occasionally.
07 - Top each browned chicken breast with marinara sauce, shredded mozzarella, and Parmesan cheese. Bake in preheated oven for 15 minutes or until cheese is bubbly and internal temperature reaches 165°F.
08 - Bring a large pot of salted water to a boil. Cook spaghetti until al dente according to package instructions. Drain thoroughly.
09 - Divide spaghetti among plates, spoon marinara sauce over pasta, place one chicken breast on top, and garnish with fresh basil and extra Parmesan if desired.