In a medium pot over medium heat, heat oil. Add onion and garlic, stir to combine, and reduce heat to low. Cover and cook, stirring occasionally, 10 minutes. Uncover and cook, stirring occasionally, until onion is translucent, softened, and fragrant, about 5 minutes more.

Stir in tomatoes and basil. Bring to a boil, then reduce heat to medium-low and simmer, stirring occasionally, until slightly thickened and flavors have melded, about 30 minutes; season with salt and pepper. Discard basil.

In a shallow bowl, combine panko, garlic powder, and 1 cup Parmesan. Into another shallow bowl, pour flour. In a third shallow bowl, beat eggs with 1 tablespoon water.

Pat chicken dry; season with salt and pepper. Dip into flour, shaking off excess, then into egg, then into panko mixture, gently pressing to adhere.

In a small bowl, mix mozzarella and remaining 1 cup Parmesan.

Heat broiler. In a large, heavy pot over medium heat, heat oil until a deep-fry or instant-read thermometer registers 350°; if it smokes a bit, that’s okay. Set a wire rack in a baking sheet.

Working one at a time, fry chicken until one side is golden brown, about 1 minute. Flip and continue to fry until golden brown on the other side and the sound of the frying chicken gets slightly louder—this is the moisture evaporating from the chicken—about 1 minute more. Transfer to prepared rack. Repeat with remaining chicken. Spoon some marinara over chicken. Sprinkle with cheese mixture.

Broil, watching carefully, until cheese is melted and brown spots form, about 2 minutes.

Divide chicken among plates. Top with basil. Serve with remaining marinara alongside.