01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Place each chicken breast between plastic wrap sheets and pound to an even 1/2-inch thickness.
03 - Arrange three shallow bowls: one with flour, one with whisked eggs and milk, and one with breadcrumbs mixed with Parmesan, garlic powder, oregano, salt, and pepper.
04 - Dredge each chicken breast in flour, shaking off excess. Dip in egg mixture, then coat thoroughly with breadcrumb mixture.
05 - Heat olive oil in a large skillet over medium-high heat. Fry chicken 2-3 minutes per side until golden brown. Transfer to prepared baking sheet.
06 - Top each chicken breast with prepared marinara sauce, shredded mozzarella, and additional Parmesan. Bake for 15 minutes until cheese bubbles and chicken reaches 165°F internally.
07 - While chicken bakes, heat olive oil in a saucepan over medium heat. Sauté garlic and onion until soft (about 3 minutes). Add crushed tomatoes, basil, oregano, sugar, salt, and pepper. Simmer uncovered for 15 minutes, stirring occasionally.
08 - Bring large pot of salted water to a boil. Cook spaghetti according to package instructions until al dente. Drain well.
09 - Divide spaghetti among plates, spoon marinara over pasta, place chicken breast atop, and garnish with fresh basil and extra Parmesan if desired.