Pre-heat the oven to Gas Mark 4, 180°C (350°F).
Heat a large frying pan with half the oil and cook the bacon and onions over a low heat for 2-3 minutes until golden.
Put 2 large pieces of tin foil on a large roasting tray, making sure they overlap a little. Cover with greaseproof paper. Spread the onion and bacon mixture in the centre and place the lamb on top. Season with salt and pepper.
Mix the lemon zest and juice with the garlic and brush it over the lamb. Arrange the bay leaves and herbs around the lamb, seal with the paper and foil and place in the oven. For medium lamb allow 25 minutes per 500g plus 25 minutes and for well done lamb allow 30 minutes per 500g plus 30 minutes. For the last 20 minutes open the package and fold back the foil and greaseproof paper to allow the lamb to brown.
Remove from the oven and leave to rest for 10 minutes, then slice and serve with the bacon mixture.