Put the beef mince, onion, 1 tsp oregano and half the garlic in a bowl; season. Mix with clean hands to combine, then roll into 16 balls.
Heat 1 tbsp oil in a large, deep frying pan over a medium-high heat and fry the meatballs for 6-8 mins until golden brown all over. Preheat the oven to gas 7, 220°C, fan 200°C.
Pour the sauce into the pan and stir to coat the meatballs. Cook for 3-4 mins until bubbling. Mix the remaining garlic, ½ tsp oregano, the bread and cheese in a bowl.
Stir the chopped basil and chilli, if using, into the sauce, then transfer to a roasting dish. Scatter with the cheesy croutons and drizzle with ½ tbsp oil. Bake for 15-20 mins until the topping is golden and crisp and the meatballs are cooked through.
Meanwhile, boil the green beans for 2-3 mins until tender. Drain and serve with the bake, scattered with the whole basil leaves.
