Pour the vinegar into a small saucepan. Bring to the boil over a medium heat, then reduce the heat and simmer for about 8–10 minutes until thick and glossy. Set aside to cool slightly, but keep warm.
Place the monkfish fillets on a chopping board and, using a sharp knife, make a deep incision about 5 cm (2 inches) long in the side of each fillet. Stuff with 1 teaspoon tapenade and 2 basil leaves. Wrap 2 bacon rashers around each fillet, sealing in the filling, and fasten with a cocktail stick.
Bring a saucepan of salted water to the boil, add the green beans and cook for 3 minutes, then add the peas and cook for a further minute. Drain and keep warm.
Heat a griddle pan over a medium heat and place the monkfish fillets in the pan. Cook for 4–5 minutes on each side until the fillets are cooked. Set aside for 1–2 minutes.
Meanwhile, toss the beans and peas with the spring onions, feta and basil oil, then arrange on serving plates. Top with a monkfish fillet and serve immediately, drizzled with the warm balsamic dressing.