• 2 to 2 1/2 pounds Yukon gold potatoes, cut in large chunks
• Kosher salt
• 3 tablespoons unsalted butter, divided, room temperature, plus more for serving (optional)
• 3 cups shredded savoy cabbage or lacinato kale leaves with ribs removed
• 4 scallions, sliced, white and green parts separated
• 3 cloves garlic, grated
• 1 teaspoon dried mustard, such as Colman’s
• 1/2 cup whole milk
• 1/2 cup chicken stock (or more milk)
• 1/2 cup packed finely grated sharp Irish cheddar cheese, or more to taste
• Freshly ground black pepper

Place the potatoes and 2 teaspoons salt in a pot and cover by 1 inch with cold water. Bring to a boil and cook until the potatoes are tender, 15 to 20 minutes. Drain thoroughly, then return to the pot.

While the potatoes are cooking, heat 1 tablespoon butter in a large skillet. Add the cabbage or kale and the white parts of the scallions. Sauté until the greens wilt, 3 to 4 minutes. Add the garlic and stir until fragrant, about 30 seconds more. Remove from the heat.

Add the remaining 2 tablespoons butter and the mustard to the drained potatoes and mash with a fork or potato masher. Stir in the milk and chicken stock, then stir in the greens, and the cheese (if using). Season with salt and a generous grind of black pepper.

Serve warm with an additional pat of butter on top (optional) and garnish with the green scallions.