Put potatoes and 2 teaspoons salt in a large pot. Cover with cold water.
Bring to a boil over high heat then reduce heat to a simmer.
Place the chopped kale into the pot and add the sausage links on top.
Prick the sausages with a knife to prevent bursting.
Meanwhile, in a heavy skillet, heat 2 tablespoons oil over medium-low heat.
Add onion and cook, stirring occasionally, for 6-7 minutes, until translucent.
Add garlic and cook for 30 seconds.
Once the sausages are heated through, remove and set aside. Continue cooking potatoes and kale until tender.
Scoop out a cup of potato cooking-water and set aside. Drain the potatoes and kale and return them to the pot.
Add butter, milk, 2 teaspoons salt, vinegar, ½ teaspoon pepper, and the cooked onions and garlic mixture.
Mash potatoes and kale together. For creamier potatoes add potatoe cooking-water, a little at a time, stirring, until you get the desired texture.
Divide the kale-potato mash between 4 or 5 serving bowls. Drizzle a teaspoon of olive oil or melted butter per bowl and sprinkle with chopped scallions, if you like.
Arrange sausages pieces on top. Dip a forkful of the mash and sausage mixture into Dutch mustard. Enjoy!