Preheat the oven to 200C/400F/Gas 6.
To make the lamb, place the breadcrumbs, melted butter and mustard into a small bowl and mix together well, then season with salt and freshly ground black pepper.
Spread the mustard mixture over the fatty side of the lamb rack and place onto a baking sheet into the oven to roast for about 18 minutes (for medium rare), or until cooked to your liking. Remove from the oven and rest for two minutes.
To make the pea ragu, melt the butter in a frying pan over a medium heat. Add the garlic and leeks and gently fry for 2-3 minutes, until soft.
Add the peas and cook for three minutes.
Increase the heat, add the white wine and lemon juice and reduce the liquid volume by half.
Add the cream and fresh mint and cook for 2-3 minutes, until thickened, then season, to taste, with salt and freshly ground black pepper.
To serve, spoon the pea ragu onto a large warm plate and top with the rack of lamb.