Peel the potatoes and boil them for 15 minutes in deep, salted water.
Set the oven at 200C/gas mark 6. Peel and halve the shallots lengthways. Pour the oil into a roasting tin, and add the shallots, cut side down.
Drain the potatoes then put them in among the shallots. Place the sausage among the potatoes and shallots then bake for about an hour till the shallots are golden and soft, and the potatoes are lightly crisp.
Grate the cheddar. Remove the onions and sausage to a warm place, add the grated cheddar to the potatoes in the tin, a little salt and pepper, then roughly crush with a potato masher to give a coarsely textured, crusty mash.
Fold the shallots and parsley through the roast potato mash, and serve with the sausage.