2Wash, dry, and halve Brussels sprouts; transfer to a baking sheet pan.
3Slice sausages into ½-inch-thick rounds; add to the baking sheet.
4Drizzle sprouts and sausages with oil and sprinkle with salt and pepper. Place in the oven (it doesn't have to be fully heated) and roast until sprouts are tender-crisp and sausages are cooked through, 18-20 minutes.
5Meanwhile, fill a medium saucepan about halfway with hot water (from the tap); add salt, cover with a lid, and bring to a boil over high heat.
6Wash, peel, and chop the potatoes into ½-inch cubes. Add to the saucepan and cook until very tender, 7-10 minutes. Drain and return to saucepan.
7Meanwhile, combine oil, Dijon, mayo, vinegar, sugar, salt, and pepper in a small bowl; whisk until smooth.
8Add butter, milk, salt, and pepper to potatoes and mash until smooth.
9To serve, divide potatoes between bowls, top with Brussels sprouts and sausages; drizzle with creamy Dijon dressing and enjoy!