Preheat the oven to 180C/350F/Gas 6.
For the roasted monkfish, drizzle a little olive oil over the monkfish tail. Mix the crushed peppercorns, rosemary and salt together in a bowl, then press onto the monkfish. Heat a little olive oil in an ovenproof frying pan, add the monkfish and fry until golden-brown on all sides. Transfer to the oven and roast for a further 2-3 minutes, or until just cooked through.
Heat a little olive oil in a saucepan, add the onion and fry until softened. Add the balsamic vinegar and fry for another minute. Add the cooked Puy lentils and a splash of water and cook for 1-2 minutes. Season well with salt and freshly ground black pepper.
For the roasted butternut squash, place the butternut squash onto a baking tray. Drizzle with olive oil, season with salt and freshly ground black pepper and scatter over the garlic cloves. Place in the oven and roast for ten minutes, or until tender and golden-brown.
To serve, pile the lentils onto a serving plate, scatter with the roasted butternut squash and place the roasted monkfish on top.