Spread flour on a shallow plate. Beat eggs in a bowl. Pour bread crumbs into a shallow bowl. Dredge 1 chicken breast in flour; shake off excess. Dip into beaten egg. Lift up so excess egg drips back into the bowl. Press into bread crumbs to coat both sides. Place the breaded chicken breast, unstacked, onto a plate. Repeat with remaining chicken breasts.

Melt 2 tablespoons butter in a large skillet over medium heat. Fry breaded chicken breasts, two at a time, until golden brown, 4 to 6 minutes per side. Transfer to a plate. Repeat with remaining butter and chicken breasts.

Layer chicken breasts in a slow cooker, covering each layer with marinara sauce. Cover slow cooker.

Cook on Low until flavors combine, 2 1/2 to 3 hours.

Preheat the oven to 325 degrees F (165 degrees C).

Place chicken breasts, side-by-side, into a baking dish; spoon additional sauce on top. Place 2 slices mozzarella cheese over each breast; sprinkle Parmesan cheese, oregano, basil, and garlic powder on top.