With St. Patrick's Day looming, let's whip up some champ, a rich and rustic Irish dish of mashed potatoes flecked with scallions and topped with butter. It couldn't be simpler, but as always, the devil is in the details.

Let's start with a champ primer. The dish is a relative of the better known colcannon, and both dishes - served as sides to a variety of meat dishes - date back at least several hundred years. But while colcannon adds kale - or sometimes cabbage - to the mashed potatoes, champ uses scallions or leeks.

The classic potato to use in this dish is the russet, also called the baking potato. Here in the U.S., the most famous russet is the Idaho. High-starch and thick-skinned, russets cook up fluffy and light. If, however, you're looking for creamier texture and a more intense potato taste, you ought to go with Yukon Golds. A cross between a baking potato and a boiling potato, Yukon Golds happen to make tasty champ.

Whichever potatoes you choose, don't peel and cut them before cooking. You want to boil them whole in their skins. Otherwise you end up with water-logged potatoes. And don't worry about needing to peel a hot potato. You won't burn yourself if you hold the potato with a clean kitchen towel and peel it with your other hand.

Now, on to the flavorings. It's not enough to simply mash scallions into the potatoes. You want to draw out as much flavor from those oniony greens as possible. To do that, we steep them in milk for a bit. Start by adding the chopped scallions to a saucepan of cold milk, then bring the whole thing to a boil. Once the milk boils, you pull it off the heat and let the scallions steep while the potatoes cook.

Half a cup of milk brought my champ to the degree of firmness I liked, but you're welcome to add more if you want a looser texture. Likewise, you can make the dish richer by using cream instead of milk.

Make sure the butter is at room temperature, not cold, when you mix it in. You don't want it to cool down the potatoes. And by all means, finish your champ the proper Irish way with an extra little pat of butter right in the middle of the mound of potatoes. This allows you to dip each bite of the potato into the butter.

Sara Moulton was executive chef at Gourmet magazine for nearly 25 years, and spent a decade hosting several Food Network shows. She currently stars in public television's "Sara's Weeknight Meals" and has written three cookbooks, including "Sara Moulton's Everyday Family Dinners."

Start to finish: 1 hour (25 minutes active)

Servings: 4

2 pounds russet or Yukon Gold potatoes, scrubbed but not peeled

1/2 cup whole milk

1/2 cup finely chopped scallions (white and green parts)

1 teaspoon kosher salt

4 tablespoons (1/2 stick) unsalted butter, cut into pieces, at room temperature, plus extra

Place the whole potatoes in a medium saucepan. Add enough cold salted water to cover by 2 inches. Set the pan over high heat and bring to a boil. Reduce the heat and simmer, uncovered, until the potatoes are very tender, about 20 to 40 minutes, depending on their size.

Meanwhile, in a small saucepan over high heat, combine the milk, scallions and salt. Bring just to a boil, then immediately remove the pan from the heat and set aside to steep.

When the potatoes are tender, drain them and let them cool briefly until you can peel them. To do this, hold each potato in a kitchen towel while peeling. Peel the potatoes, then place them in a medium bowl. Using a potato masher or large fork, mash them to the desired consistency and stir in the milk mixture and 4 tablespoons butter. Mix well, then season with salt and pepper.

Spoon some of the champ onto to each serving plate and top each portion with a pat of butter.

Nutrition information per serving: 320 calories; 110 calories from fat (34 percent of total calories); 13 g fat (8 g saturated; 0 g trans fats); 35 mg cholesterol; 48 g carbohydrate; 4 g fiber; 4 g sugar; 5 g protein; 510 mg sodium.