Place the potatoes in a large saucepan with 1 tablespoon of salt and enough water to cover the potatoes. Bring to a boil and simmer for 20 to 25 minutes, until the potatoes are very tender. Drain the potatoes in a colander and return them to the saucepan. Add the butter, milk, and crème fraîche and beat the potatoes in the pan with a handheld mixer until very smooth and creamy. Add a little extra milk if they're too stiff. Mix in the Dijon mustard, whole-grain mustard, dry mustard, 1 tablespoon salt, and the pepper. Place the potatoes in a heatproof bowl set over simmering water and keep hot for up to 30 minutes, until ready to serve. (You may need to add more hot milk from time to time.)

Meanwhile, prepare a grill with hot coals or heat a gas grill to medium-high and cook the sausages for about 5 minutes on each side, until browned and cooked through. Transfer to a plate, wrap with aluminum foil, and allow to rest for 5 minutes.

To serve, mound a generous portion of potatoes on a dinner plate and top with grilled sausages that have been cut in half diagonally.

Enjoy your Bangers and Mustard Mash!