Place a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons, and a half inch of water. Prick the sausages and place them into the pan. Cover and bring the liquids to a bubble then reduce the heat to medium-low and simmer the sausages in the water until cooked through, 8-9 minutes. Remove the lid, let the water cook away and crisp and caramelize the skins of the sausages, 4-5 minutes more.
Remove the sausages from the pan and add the apples and onions, season with salt and pepper, and cook until tender, 4-5 minutes. Add the hard cider, brown sugar and grainy mustard, and bring up to a bubble. Add the sausages back to the pan and cook until the liquids have reduced a bit and thickened up, 3-4 minutes.