First make the mustard mashed potatoes. Cook the potatoes in a large saucepan of boiling water for about 20 minutes, or until tender.
Place the sausages on a preheated ridged grill pan and cook for 10 minutes, turning them to get an even color. Add the onion wedges and cook with the sausages for 6-7 minutes.
Meanwhile, drain the cooked potatoes well and return them to the pot. Place over a low heat and allow any excess water to steam away without coloring the potatoes. Remove from heat, then, holding the warm potatoes in a dish towel, peel them carefully. You should be able to squeeze the skins off through the towel. Place in a bowl. Mash well, using a potato masher or fork. Add the butter, wholegrain and English mustards, garlic, and salt and pepper and continue to mash. Taste the potato and add more mustard, or salt and pepper, if desired. Finally, stir in the parsley and a dash of olive oil.
Serve the mashed potatoes and sausages together with the onion wedges.