Heat the oven to 200C/fan 180C/gas 6. Bring a pan of water to the boil and cook the pasta shells for 10 minutes or until al dente and not quite cooked through. Drain, then rinse under cold water and drain again.
Heat the oil and a knob of butter in a large pan over a medium heat. Fry the onions for 5 minutes until starting to soften, then add the garlic, reduce the heat to low, and cook for 5 minutes. Season, then add the stock and 450g of the peas. Bring to the boil for 1 minute, then turn off the heat. Tip the mixture into a blender with the crème fraîche, mint and basil, and blitz to a smooth sauce.
Pour the sauce into a large, shallow baking dish. Put the white and brown crab meat, lemon zest and juice, shallot, red wine vinegar, sugar and crème fraîche into a small bowl and mix together thoroughly. Spoon into the pasta shells, then sit them in the pea sauce. Sprinkle over the remaining peas and the Grana Padano and bake for 30 minutes until the cheese is golden and sauce piping hot.