Preheat the oven to 200°C/gas mark 6. Heat 1 tsp of the oil in a frying pan and cook the onion for 4-5 minutes, until just beginning to soften. Add the garlic and cook for 1 more minute.
Stir in the fennel seeds and pour in the passata, then rinse the carton with a splash of water and stir in. Bring to the boil, then simmer gently while you prep the chicken.
Place the chicken fillets between two sheets of clingfilm and pound with a rolling pin until they're 1-2cm thick. Lay 3 pepperoni slices on top of each chicken breast. Carefully dip each fillet into the beaten egg to coat.
Stir the breadcrumbs and parmesan together on a plate, then season. Place the chicken in the breadcrumb mixture and turn over to coat well, making sure the pepperoni stays in place.
Cook the breaded chicken breasts in the remaining oil in a large non-stick pan for 5 minutes on each side. Pour the tomato sauce into a baking dish and place the chicken in, pepperoni side up. Bake in the oven for 18-20 minutes until the sauce is bubbling and the chicken is golden and crunchy.
Serve with the green beans and fresh parsley.