Fresh spring onions make this effortless sausage and potato sheet-pan supper feel elegant, but you could just as easily use scallions, leeks or nearly any other member of the allium family. The spring onions and potatoes make the perfect bed for roasting: They prop up the sausages so they brown instead of steam, and they absorb the sausage drippings while doing so. A spicy, tangy mustard relish cuts the richness of the sausage and offsets the sweetness of the spring onions. Should you have leftovers, they are excellent tucked between slices of toasted bread smeared with plenty of mayonnaise and more mustard.