This soup is inspired by colcannon, a traditional Irish dish of mashed potatoes and cabbage or kale. Buttery Yukon golds are medium-starch potatoes, which makes them particularly versatile. You could use russets or fingerlings instead. Leeks, too, have a wonderfully complex flavor and are a traditional component in colcannon. If you can’t find them, sub two yellow onions in the recipe. Yellow onions have more intricate flavor than white onions (and they tend to be cheaper). Roasting the kale enhances its flavor and yields a crunchy texture that makes it a pretty garnish for the soup. Use any variety of kale you find, from curly to dinosaur (also known as lacinato).